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3 flashes is a draft pressure switch open error. This switch must close proving there is adequate draft before the burner will fire up. One possible cause is a blocked exhaust vent.
You need to check the exhaust vent for blockages. Another possibility is that the small diameter vacuum tubing connected to the switch (on top of the heater) is kinked, disconnected or has water in it (should be dry). Beyond that assuming the exhaust fan is working, replacing the switch is the next step.
The pressure switch is a sort of disk shaped object (see attached pic for typical)
You seem to be describing the exhaust fan assembly. The vent is the pipe connected to it. You need to start where that pipe terminates outdoors. I gave you a pictures of a typical pressure switch and it has a small diameter tubing connected to it. It's on top of the heater attached to the fan assembly.
The only place you can buy a switch is a plumbing supply house hat sells Rheem and they usually won't sell to an unlicensed person. Your best chance is to check out the other possibilities I mentioned before. I need the model number of your unit
Have you checked where the vent terminates outdoors? No the switch tubing is not PVC the vent is PVC.
Looks like there is an access panel you have to remove to get to the switch on this model. If the tubing to the switch isn't kinked or full of water then it could be the switch. If the unit is less than 6 years old it may be under warranty.