The problem is in the thermostat wiring, likely it is grounding somewhere between the thermostat and the air handler/furnace unit.
The problem is most likely in the thermostat wire between the air handler and the thermostat or even outside at the AC condenser unit. the 3-amp fuse is protecting the circuit board from burning out. I recently saw this on a furnace blowing a 5-amp fuse due to the AC compressor had the thermostat wire running through a chassis knock-out hole with no protection. The thermostat wire was grounding out to the metal chassis. I detected this by testing each thermostat wire at the furnace circuit board for resistance (Ohms to ground). Remove each wire one-at-a-time and test that wire with one meter probe while the other probe lead is grounded. If I recall correctly the Ohms on the faulty (grounded) wire was less than 5. Using Ohm's Law formula v=iR and a 24v control voltage you can see that 24/5=4.8 amps. However, the control voltage actual RMS on my meter was near 29 volts, so the fuse would definitely blow for a leak to ground with 5 Ohms resistance.
Thank you. I had discovered that it was the thermostat wire prior to reading your answer but you were exactly right. I put on a new thermostat wire and it's working great
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SOURCE: We have a Frigidaire Model
Refer page 30 in this manual for wiring diagram. It shows the connections between the thermostat and the control unit. Hope this helps.
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