'Working on a 10-yr old Goodman GMP050-3. A/C is okay, but no heat. Circuit board and ignitor were just replaced and the flame sensor has been cleaned.

The unit comes on, status light solid, ignitor glows, gas comes on and ignites, but after 2-3 seconds the 3A fuse on the board blows.

I hate to say this but the circuit board could still be bad. Look at all the wiring that goes to the board for any wear area an make sure none of the components on the back side of the board are shorting out when it was installed. Usually the fuse blows when there is a problem with thermostat wiring or another component that is attached to the board.

The next time you put in a new fuse use a 5 amp fuse, sometimes The coil in the gas valve plus any other coil that might be using amperage could add up to just a bit more than the 3 amp fuse can handle. Are you using an air cleaner or anything that could use more amps.

If the coil on the gas valve is going bad it will do the same thing. One thing you can do is check the amp draw of the valve when it comes on. If the amps start to climb above the amp rating of the fuse you may have a bad gas valve. You can disconnect one wire on the valve and call for heat if the ignitor glows and then goes out and tries again with out blowing the low voltage fuse. The problem is in the gas valve

With the gas valve disconnected the ingitor will glow, then stop, then glow again. The gas valve does seem to make a buzzing noise before blowing the fuse. So the gas valve is bad? Any chance the limit switch is bad? (Because I'd want to replace it at the same time since it's conveniently located right behind the gas valve.)

Sounds like the coil is shorted inside the gas valve. The only thing you can do is replace it. If the limit switch was bad the hot surface ignitor would not get power to glow.