An update was reelased for Left 4 dead today. One of the things it seems to fix is the 3 infected only bug:
The most noticeable things that the sv_cheats fix changes are the melée "cooldown" (that isn't even a cvar anymore AFAIK) and the only-3-bossinfected-at-a-time-so-one-is-stuck-spawning-for-minutes problemQuoteA required update for Left 4 Dead is now available. Please run hldsupdatetool to receive this update. The specific changes include:
- Finding a dedicated server when starting the game from a lobby is now faster and more reliable
- Versus mode is now locked to Normal difficulty
- Difficulty on a server is now locked to the one set by the lobby reservation
- Fixed lagouts/spikes after level transition
- Fixed directory issue with spray logs in some cases
- Fixed video settings not saving after closing video options dialog
- Corrected a boomer suicide exploit
- Several minor localization fixes
- Fixed a error were disabling cheats would incorrectly reset data
Jason
A bug-fix patch expected 'this week'.
http://forums.steampowered.com/forums/showthread.php?t=784064