I was reading service manual on similar WH with same gas control and blower.
Same vent requirements. Except if vent distance to first elbow is less than 12″, then total horizontal distance is reduced.
Page 13 service manual shows: 3-flashes: pressure switch or blower temperature switch not working – open position.
It lists 6 causes:
1. Vent blockage or improper vent configuration. (Which is being blamed in your case)
2. Pressure switch tubing kinked or blocked.
3. Faulty pressure switch.
4. Blower not spinning up to speed.
5. Blower temp or exhaust temp too high.
6. Faulty blower temperature switch.
If same problem occurs when unit is venting directly into garage, that would indicate a problem before the vent gas is vented, instead of vent.
I’m sure the plumbers checked if electrical outlet is wired correctly. Best would be a dedicated electrical outlet with nothing else connected to line.
Absolutely if this was happening to the CEO of Rheem, they would immediately replace the WH instead of inconveniencing the guy.