Sorry, I don't know of any labs that can do it in Sg.You will have to call around - try Ruby Photo. B/W in Sg is usually done by private individuals freelancing for the photo stores.
Failing that, perhaps a club member of the SAFRA photographic club can help you - SAFRA have their own darkroom - the enlarger's head may have to be reversed to project the image onto the floor to get to A3 size.
Mind you, the resulting prints will have to be viewed at a certain distance and not up close, as you can imagine they will be grainy.
