Should I do voluntary contribution to all my 3 accounts CPF even though I will use the OA for payment of BTO in the future?

Good answer from Chang. Just to add on: I would just do enough to meet My goal for 1M65 in SA/MA. But a HDB flat down payment is important. So make sure you have enough for that first, either in cash or CPF.

What is the purpose u are trying to achieve?

No, I wouldnt recommend doing that. As contribution this way, you will not receive tax relief (unless u are self-employed). If your purpose is to take advantage of the extra 1% rate return for the first $60k balance or for retirement planning, you will need to topup to the special account only. It is called the "retirement sum topping up scheme".

Do chat me up if you would like to discuss more.