Group practice is just that..."Group" practice. It is not the time, nor the place to be working on individual parts, but rather putting the individual parts together...seeing how they fit, making alterations to the song structure, etc. AS A GROUP.
Having said that, 2 to 4 hours is plenty of time to work out new songs or tighten up songs that still need work.
Through the years, I've found that too many musicians do not do enough of their own work and when it comes to group practice, they waste everyone's time trying to figure out their part at that time.
Fortunately I never played with those types for very long. The best practices are the ones where everyone knows their parts and the songs are just worked out together. When everyone knows their parts, there is more time to work on more constructive things like arrangements, set lists, song to song flow, new key changes or parts, etc.
If you are practicing for three hours and you are not getting tighter...then it is not a matter of how much you practice, but rather how you practice.