Dimmer: plug it into the wall, plug the mat into the dimmer output, adjust dimmer to dial back the heat. You will need 2, one for each matt. Shouldn't cost more than $25 for both at Lowes or Home Depot.
Thermostat: plug it into the wall, plug power strip into thermostat, plug heat matts into power strip, put thermostat probe under a matt, set thermostat to 90*F, wait for an hour, adjust up or down as needed to get a surface temp of 90*F.
Also three inches of substrate over the heat pad is way too much, 1/4-1/2 inch under the warm side hide is plenty. You can leave it thick on the cool side so your snakes can burrow if they want.
Heat Gun or IR Thermometer: buy one so you know the surface temp of the tub over the heat matt.
A vet bill for a burn will run $150-300 depending on the cost of living in your area.They also seem pricy for the good ones.