P0300, P0301, P0303 Codes 2014 Toyota Corolla S+

Maybe fuel or exhaust flow related, did you run a fuel pressure/flow test and a exhaust back pressure test

I don't have the tools for a proper fuel pressure test, but there's fuel in the rail and it has pressure. I don't know how to check the exhaust pressure.

A fuel pressure tester can be purchased but good one are expensive. A DIY back pressure tester is fairly cheap from most any parts store.

So fuel flow means fuel filter and/or pump. But what's the fix if there's something wrong with the exhaust?

Catalyst, muffler or crushed exhaust pipe.

Compression test cylinders 1 and 3. If they're next to each other I'd suspect a head gasket blown between the two cylinders. The compression test will tell us. Hope that helps! Jim