I realise that I can get prestige tokens in MW3 for having previously prestiged in MW2/BlackOps, but after playing MW3 for a while can I now go back to the earlier games and prestige in them again to get more prestige tokens?
You don't get prestige tokens for each prestige rank you have. You get a prestige token from having gone prestige in a previous game (so 2nd prestige in MW2 buys you no more prestige tokens then 10th prestige).
However, if you haven't gone prestige in a previous game (Modern Warfare, MW2, World at War, or Black ops), you can go back and prestige to gain an additional token.
Hope this helps!