bedroom(rear)furnace ignites for about 3 seconds then shuts off, mid and front furnance works fine along with hot water heater,,,, Capt Bob

Not a mechanic nor rocket scientist, but it sounds-like it could be the CONTROL BOARD or THERMOCOUPLE.

Does it actually fire-up and make and blow heat -or- does the fan just run for a short period, then shut-down???

I think one of mine just ran for a while [fan], but never fired-up, then shut-down ...the control board was the culprit.

Check for dirt daubers in the burn tube

A boring exposé of how those things work.

There's a time-delay relay that's the first thing to energize. It activates the control board and the fan. The fan, with enough pressure, operates a "sail switch" (activated by a damper-like thingy that opens in airflow and contacts the microswitch), which signals the control board that there's enough air circulation for safe ignition, and then the control board initiates that sequence. When the thermostat cuts off the flame, the time-delay relay runs the fan for 20-30 seconds to cool the hotbox.

Because the unit is shutting down in about 3 seconds, that would suggest the sail switch is bad, causing the control board to shut the system down. It's inexpensive, but the unit has to be pulled to access it. RV dealers have a tester for control boards, and most will test yours for free. Check the control board, but I suspect the sail swtich or its connection is bad.

it does ignite for a few seconds and then shuts off flame, fan still runs good, we just fired up furnance for the first time at the GTG, worked fine for a couple or three times then strted acting up, i will try everyones suggestions and let you know.. Thanks Capt Bob

This sounds-like what mine had been doing

...worked fine [other-than the squeeling motor] ...then statrted the intermittent thing that you describe ...then would not "fire" at all any longer [the fan would just run for a bit, then shut down].

Unfortunately, I had the motor replaced just prior to the control board failure.

...it's always something with $$$ attached to it.

It could be the thermocouple - I'd try a known good one from another furnace. If not that, try another control board. If the thermocouple doesn't transmit a "heat-detected" signal to the control board, it shuts down the gas.