Confused on valve config for Heater and FL3 error

Sorry that nobody has responded. Could you post a better picture of your setup? The one you posted is literally about the size of two postage stamps on my computer screen and gets blurry when I try to blow it up.

While the bigger picture helps see those valves, it doesn't show where the pipes run to/from.

Can you post pics from a few different angles and an overall picture of the equipment. That may help us get a better idea of what you have. Also, move stuff out of the way so we can see valve positions, etc.

Looking at only the three red handled valves on the left....
In their position in the pic in post #10, water is flowing from the filter through the heater and then back to the pool. "Cold" is filtered water entering get the heater - "Hot" is water exiting the heater on its way back to the pool. The unmarked valve is a heater bypass - it sends filtered water back to the pool, bypassing the heater.

If you open the closed valve and then close the two open valves, you will bypass the heater. If not heating the pool, I'd suggest bypassing the heater - you should get better flow back to the pool. But I would also suggest running water through the heater for a few minutes once every week just to exchange the water in the lines. Also, don't try to run the heater with the Hot/Cold valves in the closed position.

The two valve on the suction side of the pump seem to indicate one is for the skimmer (S) and the other for the main drain (D).

Not sure what the other red handled valves on the right, near the window, are for. They are on the return side and may adjust the returns, a water feature, etc. You can try turning one off or partially off and checking all the pool returns to see if it affects them - then open it again fully and do the same to the other valve. Just don't shut both off if pump is running or you intend to run the pump.

Can you tell us where the small black line thats connected after the salt cell goes? I see it runs to the left, but can't tell where it terminates.

In the last picture the valve alignment looks right for going through the heater, in your first picture the heater bypass valve is open in addition to the inlet and outlet valves. If you were trying to run with all three valves open this would have bypassed a significant amount of water around the heater.