If I'm rolling dice in King of Tokyo, I get what I want after 2 rolls, and I choose not to take my 3rd roll. Another person has a card that says they may make someone reroll a die once each turn. If they make me roll a die after those two rolls, and I don't like the result, may I then take my 3rd roll?
I assume you are talking about Psychic Probe as the card forcing the reroll.
Based on the English translation of the French faq from Boardgame Geek:
Psychic probe: If you still have rerolls left, you cannot use them after a psychic probe effect (which takes place the moment you declare that you keep the dice results).