Losing deposit is the issue, for it's a sure loss of deposit for the 3rd or more contestants. If you go through Singapore election history, there's really no such thing as a meaningful 3- or multi-cornered contest. Wherever in Singapore, the electorate would back PAP plus one other as if in a straight fight and the others lose their deposits. 3rd or more corners only aid PAP score without possibility of winning or even scoring decently but the certainty of losing deposit.