If I understand your situation correctly, no, you cannot do that.

Account 1 is linked to account 2. If you refer account 3 from account 2, then 2 and 3 are linked. It means that once grouped, 1 and 2 are linked, and 2 and 3 are linked. Therefore, every account is linked to another one, and you get triple XP on all of them if all characters are around the same level. But if you put a level 70 in the group, you lose triple XP.

My advice is to just start 3 fresh characters and level them to 60 without boosting. You will be surprised how fast it goes with RAF.