i need help in assemble tree parts in a assemble.

for example i have a left part and a right part, and then a centerpart.

my objective it´s change the dimensions of the center partautomat, for that i include a coincident mate in the left part anin the right part, after that i change the dimension of theassemble but give me error in mates.

I believe that i need to tell to solidworks that center part donthave dimensions but i don´t now how to do

Can sameone help me, i think it´s simple solution but not forme.SolidworksAssemblies