Liked this i have tried builds there worked better but well not the worst i have tried +1

Sunfire capes passive no longer stack. In every game I have played I am priority target cause of Despair so thornmail is HUGE for me

I am not going to thumb up or down just suggest you avoid AP and focus on Defense and HP. Skip Thornmail because most likely they are going to go for your carries hitting them not you (negating your 30% return) so a 2nd SF would increase your AOE with dispair and hp/armour (increasing your ulti stun dmg). Then also after SF#1 go for Rylai's because you will get your HP and the AP you crave plus your dispair will slow your enemies every second making your carries own them (and the earlier in game you get the ganks the better). Like you said assists are just as good. Feed your carries and dont die is best.
Just my opinion because I am new with Amumu as well and found going anything but HP and Armor I was getting records like yours above. When i focus on that order I mentioned I hardly die and get mad assists. 12 and 2 with Amumu (i dont count my drunk games =P ). Final note, i dont play 3v3 often so ill have to test drive your strat to see if it works better than my 5v5 build.