Current limiting resistor on peltier element
If the Peltiers are rated at 12v/6A, then I would try measuring the current taken by 3 in series. If you can tolerate that your resulting current isn't exactly on target, then job done!

This is far more efficient than burning excess power in resistors, especially as you're starting from an SMPS. 

While the Peltier current varies a little with its thermal conditions, they are so inefficient that it's not unreasonable to model them as a resistor which changes a bit. If you can turn the voltage down on your SMPS, this will reduce the current. If not, then a DC-DC buck converter is reasonably inexpensive, if you want finer control of the current than just splitting the voltage across several in series.