You must have the rings that are thrown in, when you buy a T3. I am not a big fan of those rings. You may want to consider the Tikka Opti-Loc ring mounts for 1" tubes. Or if budget is an issue, Warne steel "Maxima" makes a decent set of rings for your Tikka. If you get the Opti-Loc ring mounts, low will be good for a 42mm objective. If you get the Warne rings, medium would be the size to get. If you only have a 1/4" of space with a 32mm objective, than your going to have to buy new rings, cuz, a 42mm won't fit with the rings you have.
Resources:
Tikka/Sako Opti-Loc: dnrsports.com you may have to call them, they have a new web site, which may still be under construction.
Warne Maxima for Tikka: Brownells.com or midwayusa.com
Good luck