I don't know if you have enough DPS in the beta to kill the third hunt in the 20 minutes though. I haven't researched it, but I was carried by 3 veterans.
The monsters scale according to whether you're in solo or in a group, and in the full game if friends join mid-fight the monster will immediately scale up to higher multiplayer-level stats, but not scale back down if they leave. I don't think the game scales according to how many other hunters are with you though.
From my experiences in the beta playing both solo, and with a group of friends who I played MH3U a ton together, two of us couldn't kill Anjanath (the 3rd quest) monster before the 20 minute limit was up on multiple attempts. With 3 people we did it, and with 4 much more easily. Solo was much easier than I was expecting though, and I had no problems finishing the hunt, but I also generally knew what I was doing from past MH experience. The monster did far less damage, and took much less to kill. Would be much more difficult solo for a new player I imagine though.