Hi Biamasser,
Welcome to flyertalk!
This might be better on the United airlines forum.
If you have bags they should be checked through to SQ. ( you may be able to get your SQ boarding pass at MCO checkin)
http://www.flysfo.com/web/page/as_airportmaps.htm
United arrives at T3 and SQ leaves from the G gates at the Intl terminal.
There is a connecting corridor between T3 and G so you do not have to reclear security again at SFO.
Assuming your flight from MCO is on time ( or even slightly late ) you will have plenty of time to connect.
Regards
TBS