To read data from a text file into a tree all you need are the following two lines

  TTree *myTree = new TTree("mytree","data from ascii file");
  Long64_t nlines = myTree->ReadFile(filename,"col1:col2:col3");

If you do not give the structure, the first line of your file must contain the tree definition

input.csv

bin:hist1:hist2
1 12 13
[...]

To use this tree you then need to get the branch names

  TTree *myTree = new TTree("mytree","data from ascii file");
  Long64_t nlines = myTree->ReadFile(filename);
 
  TString col0Name = T->GetListOfBranches()->At(0)->GetName();
  TString col1Name = T->GetListOfBranches()->At(1)->GetName();
  TString col2Name = T->GetListOfBranches()->At(2)->GetName();
 
  Float_t bin,hist1,hist2;
  T->SetBranchAddress(col0Name.Data(),&bin);
  T->SetBranchAddress(col1Name.Data(),&hist1);
  T->SetBranchAddress(col2Name.Data(),&hist2);