OK, with a wooden enclosure, UTH or Flexwatt heat tape are good heating options. Since BPs do not do well with lamps as heat sources, and NO snake or reptile should EVER have a heat rock (they should destroy them all and stop selling them), UTHs are the way to go. Of course, there are radient heat panels which work very well...but are expensive.

Anyway, in a wooden enclosure the best way to go about heat is to place the UTH on the wooden floor with some holes drilled into the wood underneath for ventilation/thermostat probe placement. Then, place vinyl flooring, tiles, or ceramic tiles over the UTH and hot glue/silicone around the edges to secure. That way, you can still easily get to the UTH for replacement without going through a lot of trouble.

I'm not clear on your cage dimensions, though. Do you have three hatchlings?? And what height are you planning on making this?
Oh, and there really is no one source that would heat three enclosures, no to mention that cages that small would be impossible to regulate and have a temp gradient in. A clearer description of the cages would help.