The Challenge is agreeing a deal, once a deal is agreed a 3 month extension will pass easily. The fear of an extension is that it is a bridge to no Brexit.
Even if one of the EU27 blocks the extension as long as the U.K. ratified the deal they will just ignore the missing legislation as arrange,ents get made.
However at the moment parliament has a bare majority for a deal that is way outside what the EU will accept, so getting to a deal by end of Feb looks impossible.