With three existing accounts (A/B/C) ...
You can link two new accounts, to two existing accounts.
A -- No Links.
B -- D (new account)
C -- E (new account)
And then use A to boost with, leveling four toons fairly quickly.
Possibly do account transfers afterwards.

Or you can go with three new accounts.
A -- D (new account)
B -- E (new account)
C -- F (new account)
And then run two pairs (A/D, B/E, C/F) together as lowbies, and boost with the existing account which you are not running as a pair.

Or if you want to level 5x RAF, without boosting.
Do some form of a chain, again two of the existing accounts, with three new accounts.
A -- D (new) -- E (new)
B -- F (new)
And then run them as a 5x group, of all new/low toons.

I personally like the middle (2nd) option the best.
As you get a booster for each set.
And RAF toons spread across all of your accounts.
But options 2 and 3, will both leave you with a sixth account.