First of all, "3/4 perspective" is not a fix term regarding a 2D projection. It's more of a made-up term, used wrong 90% of the time either unintentionally, or because it's making the most sense to the person using it. The only area it's a technical term in is photography (where it refers to horizontal shifting), as far as I'm aware.
Assuming a tile size of 32x32, "3/4" (3 out of 4) would mean to have either a floor tile base size of 32x32px and a wall tile base size of 32x24, or vice versa. Technically, you could also enforce a square tile size and still make that the smaller tile size, in which case your wall tile base height would be 42,67 (which is obviously bullshit).
Note that 3/4 is sometimes used to describe RM's perspective. By all the definition you can get, that is bullshit, as it's really a 45° angle, meaning "1/2 perspective". As that is even more confusing, I urge you to not catch up on using that, and just say "my wall tiles have the same dimensions as my floor tiles"
Additionally, it's got nothing to do with angles. Remember you're projecting on a 2D surface, without any use of 3D or whatever. Anything happening is different tile dimensions. Yes, it could be converted to angles, but really, working with pixel sizes is way easier than with 67.5°