Would speed walking three miles be better exercise than jogging three miles?
They are probably about the same. Normally, running burns nearly twice the calories per mile compared to walking. But, in your question, you specifically mention speed walking and "jogging".
Jogging is not a defined human gait. Humans either run or walk. Jogging is something a paper stacking machine does to get the edges aligned. When you say ‘jogging', you probably mean very slow running and the calorie burn rate for very slow running can be quite low.
Similarly, ‘speed walking' can be very fast and can be a very high calorie burner. It is entirely possible for a speed walker to have a faster pace, and cover a greater distance per unit time than a slow runner. Conclusion; The two forms of exercise are likely about the same.