I have 3 separate 12-2 cables going into the thermostat box each cable contains a black, red and a ground. One of the wire comes from the power supply and the other 2 wires are for 2 separate baseboard.

My thermostat is a Honeywell RLV340. It has two black wires.
How should I wire these cables without burning my house?
:)

Connect the feed black to one black on the stat. Connect the two remaining blacks from each heater to the remaining black on the stat. Connect all the reds together, connect all the grounds together.