Survival does not have join in progress, so that is completely normal that a 3rd person was not brought in after the match started. Regarding how you get into a 3v2 in the first place, it is either one player logging out while everyone is zoning into the match, or a player is disconnected due to poor connection. During the queue time you'll notice a counter in the lower right, the match won't start loading until that shows 6/6 for Survival. Once it gets 6/6 the match starts to load and during that loading time is when players can log out or get kicked. But also once that loading starts the match is committed to starting, whether all 6 make it the match or not. The only way to avoid this happening on your side of the match is to play in a team.