What would be the likely outcome if a microbe could prevent the activation of c3 in the complement cascade?

The likely outcome if a microbe prevents the activation of C3 in the complement cascade would be Inflammation. Inflammation would be the only outcome of complement activation if C3 activation is blocked. 
Complement is a system of proteins that can be activated directly by pathogens or indirectly by pathogen-bound antibody, leading to a cascade of reactions that occurs on the surface of pathogens and generates active components with various effector functions.

From a high concentration to a low concentration.