Atwood Furnace 3 red blinks

Sounds like an open high limit switch.

*** *** *** is "Ignition Lockout" on an Attwood.

There is a list of possible causes.

1: No propane
2: Bad propane valve (Solenoid)
3: Dirty burner orifice causing weak or no flame
4: Bad adjustment air damper causing weak flame
5: Bad theromcouple (or dirty you might try cleaning it)
6: Poor connection thermocouple to control board
7: assorted bad grounds.
8: Excessive (or improper) Spark gap (Very common)
9: Bad control board(This is Why I know the above)

Many Attwood furnaces use a single wire flame sensor/ignition system
In this system the thermocouple (Which generates about 1/2 volt when hot (480mV if you want to get technical, 500 is1/2) gets hit with about 1,000 volts during ignition(Based on the spark gap).

There is an electronic, sold state, switch on the mother board that protects the flame sensor chip during ignition, should this fail there is no secondary protection. I suspect mine failed and socked that senistive 1/2 volt chip with a kilovolt or more of ignition power.

I priced a new OEM board 250 bucks +S&H (This was over a year ago)
I changed undies and ordered a Dinosaur board, 108 delivered.

Opened the box and was seriously impressed by the look and feel (Quality of workmanship) Looked closer and guess what, That "Secondary protection" that is NOT on my Attwood board.... Dinosaur seems to feel it's needed, cause it's there.

Board fit perfectly (I added a pair of small zip ties just for fun but I do not think they were needed) and works like a champ, Very happy with it and do not expect to ever replace it again.

I fixed mine that was doing the same thing by removing the wire from sparking electrode, cleanig and reconnecting the wire. It was rusted.

Take a look at the post on the same topic in the last two days.