For SR3, just do it, it’s a really well-made game and runs great and you don’t need any prior knowledge except to know that it’s kind of a GTA parody. I don’t think SR1 was even ported to PC, and SR2 is pretty buggy and unstable on modern machines (though fun aside from that). SR4 supposed to be pretty great (same engine as 3 I think) but I haven’t played it.

FH4 has a healthy playerbase and I’m pretty confident it’ll still be worth playing over the next year. However beyond that as the community slowly dwindles it will eventually become less fun with fewer people doing Forzathons or seasonal co-ops or using the auctions, even if the servers are still running.