Single group playing on different busses?

You will have to write code for this. You can tweak the Audio Source after you tell it to play like this:

var result = MasterAudio.PlaySound3DFollowTransform(params);
if (result.ActingVariation != null) {
  // VarAudio gets you the AudioSource.  
  result.ActingVariation.VarAudio.pitch = 1.5f;
}

Now, regarding the rest, you are partly correct:

1) Just make a single Sound Group for each different vehicles sound (Variations).
2) AudioListener should be attached to the current player so you always hear their engine sound.
3) Assign all the vehicle engine Sound Groups (from bullet 1) to a single bus.
4) There is a new API command in the last version (4.0.4) to RouteGroupToBus, which you can use to change Bus at runtime. Perhaps you have another bus for "other players", give that a voice limit of 3. You can also turn on "stop oldest" for the bus, however there is no distance-based voice limiting feature. You may have to write code to mute / unmute Sound Groups according to distance.
5) You could experiment with adding collider triggers to other players to mute/unmute their Sound Group when intersecting with the current player's collider. That wouldn't guarantee a limit of 3 but would guarantee that only those within X distance can be heard.

Multiplayer: we have zero multipler features right now. You'd need to be coding all that yourself, although we are working on it (with Photon first).

.VarAudio.pitch is adjusting the pitch of a particular AudioSource for a particular engine. We do not share AudioSources at all. We don't use PlayOneShot ever, because you lose fine control when using it.

Even the Playlist Controller has 2 Audio Sources so crossfading can work properly.