How much memory does the iPhone SE 3rd gen have?

Storage and memory are 2 different things. Storage will depend on what you select when you buy it. As mentioned you can select either 64, 128 or 256GB. Entirely your choice. Each size will come with a price bump.

Memory (RAM) is fixed at 4GB.

The specifics were noted. Apple doesn't specify exactly what kind of flash storage is used (e.g. tri-level or quad-level flash).

As noted, the RAM is fixed, although Apple never specifies how much or what type in any general technical specs. But some have determined that it is 4 GB.

64, 128 or 256GB. See: iPhone SE - Technical Specifications - Apple

You can identify it by the model / part number that starts with an M and looks like M####LL/A.

Or by how much you paid for it, if it was a single payment rather than a a monthly subsidized service contract.