TEC1-12706 Peltier run from diffirent voltages
Stacking Peltier elements to achieve more heat pumping capacity is a little like bringing fuel along up in a rocket (fuel weighs down the rocket, i.e. there are diminishing returns (rocket equation)). 

Peltier modules are not very efficient, and generate quite a bit of internal heat, which must be pumped away by the elements below them in the stack. This effectively lowers the throughput, although at the gain of a larger total temperature difference. 

So: the 3.3 V module's gonna pump a little heat, and generate some losses. The sum of aforementioned heat and losses needs to be pumped by the 5 V module. This module in turn will generate more losses, which needs to be pumped away by the 12 V module (along with the "useful" heat transport).