Hello Amit,
I think you are mixing up a few issues.
There's no NO RULE barring any boss or employer from assigning a new assignment after submission of resignation. It's ONLY A PRACTICE that no one gives a new assignment after resignation--for purely practical reasons. One of them being the general perception [which is true in most cases--NOT ALL mind you] the individual wouldn't have any interest--as earlier--after resigning. The same reason carries for relieving earlier than the 'as-per-rules' Notice Period.
This situation could also arise if your performance was VERY GOOD while you worked there. Maybe your boss wanted to complete something that he felt ONLY YOU could do well--before you leave. I recollect I did it sometimes--in such cases, I informed the individual clearly this reason [but doesn't mean everyone should do that].
Coming to 'what to do now':
Analyse if the new assignment being given would take more than the 3 month notice period. If not, suggest you take it and put it subtly/politely in writing [e-mail?] that you will complete it--hoping it wouldn't effect your relieving Date. In a way, by this, you are pre-empting any possibility that this new assignment could be a way to hold you back OR [if your relationship wasn't smooth] to harass you. Also suggest put HR in the loop--preferably in writing.
If you think the new assignment would 'just-about' suffice the balance Notice Period, suggest you sit with your boss & bring clarity to what you 'should do' & what you 'should not do' in the available time--basically prioritize.
If you think the new assignment will definitely take much more time than available & your boss isn't listening to you, most likely the reason isn't very positive--more to hold you back or harass you. Suggest involve the HR directly--be diplomatic & assertive, 'NOT' aggressive, since you still need the Exp Letter for your future :-)
All the Best.
Rgds,
TS