[H] Running multiple AVs at once?

[Veech]

Is there any harm in running more than one AV program at the same time?  I 
have three programs installed, AVG 8.0, Spy-Bot and Malwarebyte.  They each 
take about 45 mins to 1 hr to run.  Sometimes I'll run 2 of them overnight 
at the same time.  Is there any way that doing this will cause either one to 
miss something?  Could I possibly run all 3 at the same time?