How to calculate power supply for 100x loads @ 3V 700mA
Your calculation is correct - 0.7A x 100 = 70A.

Power is not that scary - 70A at 3V is just 210 Watts. 

Some large PC (and server) power supplies are able to deliver such currents.

However you should split your power supply to few power supplies.

Easiest way to built such power supply will be traditional transformer. 

Transformer price is determined by power, not current. You can use transformer with few 7A windings (split to 10 circuits), use separate rectifiers at every block. That will reduce short-circuit current and significantly improve fire safety.

You should use fuses at all outputs or split whole power to 10-25 blocks with separate fuses, because 70A will melt thin wires when they will be accidently shorted or damaged.

You can build cheap unregulated power supply like this:



Unregulated means that voltage may vary/drop under diffrent load conditions.

Peltier cells not require very stable voltage and you can stay with unregulated supply.

Fuses are required at every transformer secondary.