just ask them if you can. from my experience some are quite okay with it. plus with the current market, ive even seen ads saying that they allow multiple tenants even up to 12 on a 3br flat! some agents can even help you find roomies
just dont sublet in secret as it will be a breach of contract and they could terminate your lease and keep your deposit.
there are also room for rent ads around property sites, that's another option
Tenent must get owner's permission to sublet or share with others. I think it's not embarassing to dicuss with this owner when you are looking for a flat. Most Sinpaporean landlord won't rent a flat and see it is shared by people from different background. But it depends and they must considert current soften market.
Actually I am just in the process of signing a contract for a flat and I am planning to find two flatmats. The landlord gives me approval but he need the legal document when new one moves in.
There are a number of options for shared accommodation. If you look under the classifieds section you will see plenty of people advertising rooms to rent - then you don't have to worry about the landlord and the agreement.