If i put my mum as occupier for bto application. Currently she owns 4rm flat no intent to sell. Can i apply for new bto 3rm as a single?

No. If your mum is listed as an occupier for your bto application, the current 4rm flat that she has will have to be sold. You, as as single can only apply for the 2 rm Bto for Single

She has no intention to sell her HDB flat, you'll not be able to put her name into the BTO 3 Rm Application as she will undertake to sell her HDB when u apply for the HDB together with her(occupier)