My GM has had to do a lot of adjusting for our group as we've fluctuated between 3 & 7 people showing up.
6+ makes things a little "busy/complex" but IMHO its preferable to 3 where you suddenly find you are short in key skills or a TPK becomes a serious possibility.
A few examples I can think of this year in our Missions campaign.
3 People showed up, Mage, Mystic, Decker.
GM later told us he took the opposition from (1 Mage, 4 Adepts, + 1 Security/Runner) down to (3 Adepts & 1 Security/Runner) removing the Mage/Commander & 1 Adept from the battle so we didn't get smoked. As it is at the end if it hadn't been for our Mage knocking himself out casting FireBalls over & over I think we would have been toast.)
Couple other times 3 people show up = Mystic, Decker, Troll.
1 - Removed Mage from gangers to make them doable, IIRC.
2 - W/O extra players the Decker was forced to deal w/ an Assault Rifle armed Merc by himself w/ a Light Pistol & ended up having to burn a point of edge to escape death since neither the Troll nor the Mystic could get to him fast enough to assist with that merc after dealing w/ multiple problems themselves.
I don't even want to think about the day when the Decker misses a game.
The bulk of the group is Awakened & lack most techie skills & we have enough issues w/o a regular Rigger or Sniper type.
So basically as others have said, the only Min/Max is really what you the GM can handle adjusting to.
As for the 3 people, if you have a Decker type, a Caster type, & a Muscle type you really have covered the bulk of your bases.
Sure, some stealth, or rigging, or face skills will come in handy too, but those 3 can handle most things.