I have MW3 and am trying to play multi-player online with 3 local players. That is, me, my son and my daughter want to join in on a multiplayer match. Not matter what I do, only 2 out of 3 of us can join and play. It doesn't matter which 2 out of 3 of us play.
Are all three of you sharing one internet connection with three consoles?
I have tried setting up a party, inviting everyone to the party, inviting everyone to the game, etc. We've all excepted the invites, etc. When one of us who is in the party and shown on the right hand side of the screen of the multiplayer menu tries to invite the one person who cannot join, the message says "your friend does not have a valid session to join". What does this mean?
That means the two of you who are in a party have already joined a "game in session", but the third person who could not join your party because of weak internet connection could not join the game because it has already reached its maximum capacity in terms of players.
*See Game Modes
We are all "friends" of the other person and have X-box live accounts. Can only 2 local players play MW3 online at a time? Thank you for helping me!
No. Not at all. If you're all sharing one internet connection, the maximum your connection could handle is probably two consoles.
Remedies for this is to Splitscreen one account and another account to play solo on one console. That way technically only two consoles are connected. Another way is for the other to connect and play somewhere else.