No, if you express the three proteins as a single fusion protein, they are read off a single mRNA. Whether or not it is necessary to preserve a Met at the N-terminal end of each domain depends on how this residue is packed in the 3-dimensional structure. Normally it can be replaced by a different residue - if the folding of the individual proteins is not perturbed by replacing the initiator methionine by something like a His tag, replacement of the Met in the fusion protein should be tolerated as well.

Thank you so much! I am greatly honored to have your answer that is very helpful for my study. thanks again