Try unplugging one component at a time from the printed circuit board, until the fuse does not blow...the last item was the one with the short and needs to be replaced... if you get all of the plugs pulled from the board, and it still blows a fuse, the the board is bad.
That would be the laymans way of doing it, you will need 4 or 5 spare 3 amp fuses.
If you have a multi meter you can set it on Ohms, and unplug each item at a time and use the ohm meter to see if there are any dead shorts... that takes a bit of skill with the ohm meter.
Let me know what you think, we can go from there without a time limit, however I am off to bed shortly, if I miss you tonight I will pick up your response in the morning.
If you want to get deeper into the trouble shooting, attach a well focused, close up photo of the wiring diagram glued inside the service panel,you can use the paper clipicon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.
If there is no paper clip icon, look for the '+attach files' link at left of your send button.
I can mark it up with trouble shooting instructions. You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.
We can go from there in the morning.