TTree::Add does not exist!
To merge Trees, use the static function TTree::MergeTrees, eg
TTree *tree1, *tree2, *tree3; //pointers to your 3 Trees
TList *list = new TList;
list->Add(tree1);
list->Add(tree2);
list->Add(tree3);
TTree *newtree = TTree::MergeTrees(list);
newtree->SetName("newtree");
newtree->Write();
Rene