Which is right please?
"3 Bedroom flat for rent" or "3 Bedrooms flat for rent"
Welcome to the forum, kruska.
In future, please give your thread a title that includes the words you are asking about. 'I need to know' tells us nothing, '3 bedroom(s) flat?' would have been fine.
To remove ambiguity, we would normally use a hyphen:
3-bedroom flat to rent.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.