1st one is the best one.
try using stocks if your a beginner so you can make good avatars without stuffing the whole thing up because of the bg behind the render.
if your going to use a render make sure the bg fits in with the avatar very well and doesnt destroy to whole thing.
try putting some text on the avatars aswell because it makes a avatar a whole lot better if you put a good text on it.
the third one the scan lines you used are to revealing!, try lowwering the opacity to make them noticable but not dominate the avatar.
try getting rid of those little straight lines because it just doesnt fit in with the avatars.
dont use to many c4ds or effects for a avatar because if your a beginner you wont learn to made pure looking ones meaning no messy looking avatars and it is clean and still looks amazing

good job anyways and i hope you take in the advice