I don't think those 3 should be synomyms of backwards-compatibility.
When you search for any of those tags, I don't expect to see anything younger than the ps2 era... maybe because I'm old.
As I re-read myself, I'm wondering why they even exist. None of them are useful tags as they are opinion based. When is a game considered classic? Old? Retro? Those are very subjective. The youngest here might feel anything ps3 is old while the older ones might not. Also, as time goes, are we to retag questions for games that become old, retro, classic???
I don't like that and I don't feel they add any value to the questions. We don't need to know a game is retro, classic or old, that's useless as the game's tag should already give us the information on when the game was released or we can easily search that.
So this is a request for burnination!
Let's burn those 3!