Note that "I do not plan to stay more than x hours at the airport" doesn't mean anything to Immigration officers. It's like being pregnant: you are in country, or not.
While the UK has, I believe, airside and landside transit visas, Schengen doesn't. If you need to enter France to transfer, and if you're transferring between T3 and T2 you have no choice but use the CDGVAL, landside, you're going to need a short-stay Schengen visa.
Note that if this is a round-trip, you'll need another for the return flights (or a two-entry visa, if that's available).