If you go:
A -- B
A -- C

And boost them both to 60th.
You get 30 levels from B to A.
And 30 levels from C to A.
For a total of 60 levels granted.

But if you:
A -- B -- C

And boost them both to 60.
You get 30 levels from C to B
And 30 levels from B to A.
And 15 extra levels from B to A because C gave 30 to B.

So it depends if you want 75 total levels, with 45 on A.
Or 60 total levels, with all of them on A.

RAF is account to account.
So you can theoretically level several sets.
The limitation is that once you hit 60th, you don't count towards the bonus.
And that you need to be within 4 levels for the bonus too.

If you went A -- B -- C.
And were 80th, 14th and 15th.
B and C would both get RAF because they're both 59 or under, and within the level range spread.

Once your first set reach 60th...
You pretty much disregard them and start on another set.
But remember to do the granted levels before the 3 months of RAF expire.
Once you have everything boosted as far as you'd like to go, then worry about 60+.

If you are in a group with.... 28, 25 and 25.
And let's say it is Deadmines, with level 19 elites.
Going with the A -- B -- C set up, the mobs are grey to the 28th.
So the A -- B RAF link does not apply.
However, the B -- C link does apply because the mobs are green to both accounts.

Let's say for example you had started at 27, 25 and 25.
And for arguements sake the mobs were green to the 27th.
All three accounts would be getting 3x exp's.
But when the 27th became 28th, its RAF link doesn't count.

If you were to group an 80, with a 10 and 10.
Both of the 10's get 3x exp's, from anything green or more challenging (quests or mobs).
B and C are RAF chain linked, so even if the mob was grey for 60 levels to A, B/C get RAF 3x.

The only thing that is not 3x exp's is exploration experience.