Either approach can work, depending on what you want.
If you go with (A) existing, (B) new/RAF'd, and (C) new/RAF'd..

A > B > C
- In this case A can boost, because B/C will meet the requirements for RAF.
- C will give (grant) some levels to B; B will do the same to A, and B can grant levels (to A) based on levels received from C.
- A (and B) get a free month, when the account they referred pays for Month 1, and a free mount when that account pays for month 2.

A > B
A > C
- In this case, B or C could boost or you could run three new lowbie toons, one per account.
- A will receive granted levels from both B and C, there is no cascading of the granted levels.
- A receives 2 free months (one each from B/C) when they pay their first month, and two free mounts when they pay their second month.

To get the triple experience you need, all of this:
a) RAF linked accounts.
b) Grouped together.
c) In fairly close proximity.
d) Both characters at level 59 or lower.
e) The characters to be within +/- 4 levels of each other.
f) The mobs or quest to not be gray for either character.