I have a 4-way pvc tee with 2" openings all around. One will vent the shower, one the sink and one the toilet. The 4th opening will connect to the 3" roof vent. Is that 2" to 3" sufficient, or is it much better (as I suspect) to have a direct 3" opening into the roof vent pipe?
2" is a sufficient vent for what you mentioned above. If you wanted you could replace the 3" roof jack with a 2" roof jack as long as nothing else is tied into the 3". If there is something else tied into the 3" then you'll obviously need to cut in a 3x2 fitting and connect there.
There's also nothing wrong with leaving the 3" vent and connecting to it with a reducing coupling if needed.