funhaver1998Maybe 4, but 3? Not possible.
I enjoy these hypothetical's and agree with this statement more than any other so far. 4 invite players can do it and I don't think it would be difficult at all. However, once you go down to 3 players, the difficulty goes way up IMO.
I don't think 3 players can consistently defend and attack at the same time on different points. The only thing the newcomers would need to do is space out and not take large 6v3 fights. If they simply focused on capping points instead of team fights, it would be too much for 1 player to deal with to stop back caps. (for the most part)
Compare this to using 4 players, the invite guys can easily have 2 players cap a forward point and 2 players defend a back-cap. Rinse and repeat, easy win.
Only 1 player defending a backcap, even against inferior players, can be quite difficult. It's very possible to do but I don't think they succeed every single time.
The biggest problem for the newcomers would probably be capping last. Invite player superior DM, experience & positioning would be too much to overcome. I can't see any scenario where the new guys can consistently outfight and cap last against the top players to win. Maybe get a round or 2 but not win overall.
As far as class composition, the invite team would need mobility more than anything else. It has to be 2 scouts and a medic or 1 soldier, 1 scout & medic. Switching up to demoman in defensive situations would be most effective but anything involving pushing and defending at the same time would need speed above all else.