As of late, T3 often only has one employee working at a time. In another thread somewhere I've posted the concurrency rates at each spa and the number of appointments that may be prebooked when traveling in F. It's something like 3 in T5 south, and one concurrent treatment in the other spas (this ignores the chair treatments). Passengers in F can prebook one of the slots in T5 south (so you can almost never prebook two appointments at the same time) and less than 1 slot per average in the other spas (they need to leave some slots open for day of departure). So even T-28 days with a five hour connection, I've had times where as a couple we could only get one booking at T3.

Quite simply the spas are far understaffed at peak hours relative to the demand. The best option at T5 is to prebook the Be Relax spa in T5B and pay for a treatment.

i dont know, not used it yet, I'll save it for a FT Do next year ;-)

You’re luckier than me! In the spring, I had a confirmed booking at the spa both ways (t3 out t5 back) for an F ticket. Outbound they had no trace of the booking at the spa so called GGL to confirm the source of the issue and reconfirm return booking... which was instead mistakenly cancelled! I didn’t even get a ‘sorry’ from either spa or GGL line, let alone a present (would have been nice!)