Possible stupid question
 

What does it mean when a helmet says "Three shell sizes" its the agv t-2 thanks

It means just that... AGV makes the helmet in three different shell sizes. The shell size is dependent on the size helmet you purchase. Arai does the same thing. Some companies just make one size shell and use different thickness liners to get the proper size.

Three different shells is a good thing. The shell will not be too big for smaller heads.