Can someone advise whether there is an officially supported by Plex TV tuner that works with, and likewise is officially supported, the Raspberry Pi Model 3 B+.
No such thing. The Pi hat is strictly for tinkerers and needs quite a lot of tinkering to even get working. And it still has only one tuner, so you can either watch something live or record one programme. But not at the same time.
Go with the Silicon Dust HDHomerun devices. They are the only officially by Plex supported tuners.
And they don’t need any drivers whatsoever, so you can use them with whatever operating system or hardware platform you choose as Plex server.
And they come with 2 or 4 tuners each.