School holidays...? 
I agree the soft play centres are pretty pricey for three kids.
I tend to put on old clothes and go out anyway, the Bot Gardens are fine in the rain, not too swampy, and the temp is just right for running around in. Take a towel to get the worst off then straight in to the shower when we get home.
Or Ikea - wander round, buy an ice-cream for a dollar, buy some coloured paper and pencils then take it home and draw.
The cinemas often do a cheap ticket in the daytime (the one in Great World City?)
Chicken rice and a smoothie in the (basement) Tanglin Mall food centre...
The $2 shop - got a Japanese name that I can never remember - one in Plaza Singapura on one of the higher floors, and one in Ion, in the lowest basement floor. Plenty of entertainment and a full-up basket to be had for $10.
The Science Centre out towards Jurong is good too, has an outdoor water park but is mainly inside with an indoor playground masquerading as a educational area, along with experiments and hand-on stuff. Much less expensive, I recall, than soft play (maybe $5 per child, $8 per adult?) and got a McD's outside.