Firstly, the 321s and 322s run on aire/wharfedale lines as well
Secondly, it makes sense to use a 333 rather than a Diesel to aid the 321s and 322s on Leeds to Doncasters as 321s and 322s are stuggling at the moment (at least, thats what I've heard from others in the thread).
Where did you hear that the 331s will be introduced then because according to this source they're scheduled to enter service next month
https://www.railwaygazette.com/news...thern-class-331s-to-serve-leeds-from-may.html
So unless the source is credible (e.g: the northern drivers group on FB that i've kept hering about on this thread, on news sites, or you work for Northern/know someone who does) then im not believing it.
So, I guess the thing that
@jonesy3001 said about the 323s is true