Manifold all vent pipes in attic to have one pipe into roof
The required vent size is half the building drain size or 2" if your building drain is 4".

Then where you penetrate the roof you need 3" for the last couple feet, if you are in an area prone to frost.

You can certainly connect all the vents together into one vent penetrating the roof. That is a common practice. Every building requires a minimum of one roof penetration and you can have more if you wish to save pipe. Although every roof penetration is a point of possible failure in the future.

Happy Saturday!