Is there going to be a left 4 dead 3

Yes, Valve confirmed it on January 15, 2010. It is scheduled for release in late-october and early-novemeber. Valve also announced that the character from Left 4 Dead & Left 4 Dead 2 will be playable, as well as four new survivors. The past campaigns from Left 4 Dead & Left 4 Dead 2 will be placed in the game; with 6 new campaigns being introduced as well. Valve also announced that this will be the final game of the series,
because Left 4 Dead: The Movie will be released in 2011. Also Valve & NECA are going to start make Left 4 Dead action figures in mid-July of 2010.

Unfortunately no official word has been announced by Valve, but there will undoubtedly be a L4D3 some time in the future.