Like the others have posted, taking more than one med is normal. I take Three, two pain, and one muscle relaxer. For me I take one LA med and a BT med.
In your case they are using two LA meds and a SA med. Meth is usually considered a LA med along with OxyContin. So it does seem a bit strange that you are getting to LA meds. Probably OxyContin and Percocet would be just fine as 5mg of Methadone is a very small dose.
Also, the OxyContin should really be dosed TID ( three times daily) for maximum coverage. It usally only works for about 8 hours max. By dosing TID you may be able to lower or even cut out your percocet.
Welcome to the boards