Vent pipes have size requirements depending on how many fixtures they vent.
If you have a few 1-1/2" or 2" vents penetrating the roof, you could
probably combine the vents into a single 3" vent instead. But, if you
already have multiple 3" vents, you're probably out of luck.

The limiting factor on DWV pipes isn't the size or number of fixtures
it vents, but rather how big around it has to be to prevent frost-up
during the winter. a 4" vent will handle anything you're likely to
find in a normal house. What you might have a problem with is
reducing the pipe-size as you go down through the system.

I don't know what code says, though. Worst case, you can always
cluster the three existing stacks together, and wrap some
sort of cowling around them.