Along with 45 minutes of swimming for 3 days a week, what other exercises should one do to get rid of belly fat and be in good shape?Don't get fixated on 'one-activity.' It's great that you're swimming 3 times a week, but if you're going to the pool and doing the same thing 3x a week and your objective is to lose belly fat you might be wasting your time.
"Good shape" is a relative term. Cross-Country Skiers are in terrible shape for Olympic Lifting and Olympic Lifters are in terrible shape for Cross-Country Skiing. You have to decide what is important to you.
Often when people say 'get into good shape' they really mean good 'cardiovascular shape' but that is activity specific, is also doesn't necessarily have anything to do with body composition. You can be overweight and still be in very good shape in this context. People have the wrong misconception that you can't be fit and be overweight, you can.
Being in good shape in this context often has nothing to do with fat loss. Actually it's the antithesis of fat loss. Fat loss is always about doing stuff that you're not good at, stuff that you're inefficient at, stuff that you're not 'in-shape' to do. Being in shape means that you're efficient, the exact opposite of what you want in any good fat-loss program.
If you want to lose fat you should do stuff you're not good at, or some stuff you're good at, but at a dosage that takes you to the edge of your ability, well beyond your comfort zone. Get comfortable with being uncomfortable because it's your friend when it comes to fat loss. There is no overlap between your comfort zone and finding success in fat loss.
Your body needs a stimulus that is appropriately challenging, so you can't just put in the time swimming and expect to get the result you're looking for. Just showing up to work, doesn't mean you get anything meaningful or useful done right? You need to get productive with exercise, the way you might with your work life or social life.
So for example, let's say you go to the swimming pool three times a week and you swim roughly 20 laps in 45 minutes every time. Ya sure your heart rate is elevated, and ya sure you're a little tired, but it's the same thing you've always done. You may have burned some calories even, but your body is well adapted to it, and it has compensated already by making sure you eat enough on the other end to stay where you are. If you do what you've always done, you get what you've always got.
Your body will adapt to that stimulus in roughly 3-6 weeks depending on ability. This significantly lessens the impact of that particular kind of exercise on your objectives. As you adapt to a given stimulus, your body requires less and less energy to do it and you get more and more efficient at doing it.
At a certain point you need to create new challenge.
You need to introduce variety. The key is, not too much variety. Too much variety and your body won't adapt either. It's about finding your Goldilock situation on the variety spectrum: JUST RIGHT.
Don't go to the pool and do a different workout every time. For one, this doesn't give you any good benchmark to compare to in the future. You won't know if you've adapted to the training if you do something completely different all the time. For two, your body won't really know what you're trying to accomplish over the short-term.
This is where Program Planning (AKA Periodization) can be useful, but it doesn't have a to be super complicated. Though in the case of an endurance sport it wouldn't hurt if it were. You should start by making sure all 3 of those sessions are different and change them all up every month.
Just going for 45 minutes and not being mindful of what you're getting done is a really really poor strategy. So maybe one session you see how many laps you can swim in 45 minutes, and you keep track of that over a month period. The other session you do 200 m interval repeats with 1:1 rest ratios. Time them and keep track of your best 200 m times. Maybe the last session is faster, more like 25 or 50 m intervals with more like a 1:4 work to rest ratio.
Maybe you change the stroke every month to periodize over the long-term better. Maybe you use different strokes each session, but mix up the distances/speed/intervals on a monthly basis. Maybe you pick two this month, and two stroke next month and do a fast, medium, light day each week still.
Otherwise, just use the 45 minutes of swimming for more of what it would be without such a plan. Good active recovery from other training, or just a fun physical activity you enjoy. Probably not really contributing much to this objective in the long-run, but no less enjoyable, or good for your overall health. Moving period is good for you even if you're not doing it really effectively for your objectives. If your body is adapted to it though, probably not going to get much out of it in the long-term from an 'exercise-for-fat-loss' perspective. That's cool, I enjoy playing Squash, but I don't think of it really as way to 'lose fat.'
Second, don't ignore nutrition. Nutrition is extremely important in this regard, you will have poor luck in trying to reduce body fat with exercise alone. Exercise is terrible for fat loss by itself (though really really good for maintenance! - so this swimming routine would be useful maybe if you were where you wanted to be physically). American Football linemen are very fit people (don't kid yourself) and by any normal standard would often be considered obese. If you get more energy into your system than you consume, the amount of exercise is irrelevant.
See my answer to Weight Loss: What are the best ways to eliminate belly fat?
Lastly, you should start weight training 2-3x a week.
Don't think about time invested, as you seem to be with swimming. That's a poor metric. Time invested does not amount to change. This is a human bias, we have this strong belief that doing more stuff will translate into better results. Most of the time it doesn't. Doing better stuff = better results. You have to find ways to be more effective with your time and learning how to weight train and periodize/organize/plan a training regime even on a simplistic level is vital to this.
Weight training is bar-none the most effective general thing you can do for performance and fat loss. That doesn't mean that you need to go and pick up a barbell or anything necessarily though, though it can. I recommend using some external resistance, combined with bodyweight generally, but bodyweight is a good place to start until your body gets used to it. Bands can work, kettlebells, dumbbells, barbells, sandbags, weight vests, etc...etc...
For weight loss you just want to do things that are inefficient, until you get efficient at them, then switch it up to new inefficient things to do until you get good at them. Luckily, it's impossible for your body to be good at everything at once, so if you're getting strong, it's probably at the expense of some endurance, and if you're improving your endurance, it's probably at the expense of your strength. There is a continuum of opportunity cost that you can manipulate in this regard.
See my answer to Exercise: How do you customize a weight loss and exercise plan for yourself?