I just finished IMAX1. Whew... boy, anything that has that many plie jacks and power scissors in it will never be "easy" to me!!!
Here's what my HRM says for each of these...
IMAX1 - AvgHR 142 MaxHR 173 calories - 474
IMAX2 - AvgHR 142 MaxHR 174 calories - 430
IMAX3 - AvgHR 149 MaxHR 176 calories - 506
The 1 and 2 are close to the same except for the calorie burn. I did change my weight on my HRM about a week ago (down) so the IMAX1 was done on the lower weight and the IMAX2 on the higher weight. I thought that you would burn more calories with a higher weight inputted into the HRM than a lower weight??? The only other variable is IMAX2 time, according to my HRM, was 55 minutes, IMAX1, according to my HRM, was 1 hour. I believe that explains the difference. At that intensity, I believe I could easily burn 44 calories in five minutes.
Just going on perception, IMAX3 feels harder to me than the other two.