There are plusses and minuses with each choice:
IMHO Interval Max (the original) is a tad more challenging than I-Max 3; that's a plus. Also, it comes on a DVD with Rhythmic Step and MIC, two other great Cathe workouts. Another plus.
However, I-Max 3 is better chaptered (you have chapter points for step segments and blasts, whereas with Interval Max you only have chapter points that begin at each step segment, no separate chapter point that is right at the blast), and includes premixes even just using footage with that one workout; that's a plus for I-max 3. Also, I-Max 3 has a beautifully grisly blast that makes blast #9 in Imax 2 (the dreaded plie jack blast) feel like a walk in the park: you do 24 straight plie jacks then criss-cross "recovery" (NOT), then 16 more plie jacks and a criss-cross non-recovery, then another 8 plie jacks and a criss-cross non-recovery, then ANOTHER 8 plie jacks and a criss-cross non-recovery.
I vote get both.
A-Jock