Can I do a voluntary contribution to my CPF even when my SA is full, but both my OA and MA have not been maxed out yet?

As long as you have not reached your annual limit (AL), you can do a VC. However, if you do a VC and let's say you have a good bonus, anything in excess of the AL will be refunded without interest, so do your sums first.

You can get tax relief for VC that is subject to the AL if you direct it to MA specifically. CPF SA topup via RSTU is a seperate $7K cap for tax reliefs. The amount of MA-VC relief granted will be given to the lowest of the following:

1. Voluntary cash contribution directed specifically to Medisave Account or

2. Annual CPF contribution cap for the year, less Mandatory Contribution (MC)* or

3. Prevailing Basic Healthcare Sum(BHS)^, less the balance in Medisave Account before the voluntary cash contribution

Yes you still can do voluntary contribution to 3 accounts subject to annual limit of $37.7k