If all those lines aren't on contract, you can go to SingTel and ask for downgrade of the plan to MobileShare to share.
No handset for the 3 MobileShare lines, only the 1 Combo 3 line.
If not, it'll be counted as 3 new supplementary lines alongside the 3 Combo 2 + 1 Combo 3. Also, you would need 3 dual Sim handsets so to use 1xCombo 2 + 1xMobileShare.