How many watts is your 3' heater? I think they are usually around 500W which means 1500W total. This will easily fit on a 20A circuit, so you only need #12 wire instead of #10. If these are 240V heaters, you could even use a 15A circuit if you want and use #14 wire.
You'll need to get a double pole thermostat, and most baseboard thermostats are rated for up to 24 amps. This will have 4 wires -- two for the power source and two to go to the heaters. Run your power cable to the thermostat box. Color the white wire red at both ends. Connect these two wires to the input wires of the thermostat (if you're lucky it would be black to black and red to red).
The next part you have two options. You can run 3 separate cables from the thermostat box, one to each heater. Or, you run a cable to the first heater. Then from there, run a cable to the second one. And then again run a cable from the second to the third. You could even do a combination of the these.
Having all the wires in the thermostat box may be too crowded unless you have a box that is 25 cubic inches or larger (assuming #12 wire). But if you do it here again you recolor all the white wires red and put all the reds to one hot output of the thermostat (use the red if it is so colored). Combine all the blacks together too with the other hot output from the thermostat.
If daisy chaining (going power to heater1 to heater 2 to heater 3), again you connect the wire to the first heater to the thermostat output (black to black, red to red). At the baseboard heater, connect the cable from the thermostat, the cable going to the next heater, and the baseboard heater itself by matching colors (blacks to black, reds to reds).
If all the heaters and thermostat only have black wires, it doesn't really matter. You just have to realize there are two hot wires and both of those wires need to go to each device (but both wires can't touch each other, they need to be kept in two separate bundles). Color coding just helps keep things straight so you don't short out the circuit.