Why Is Tetanus Being Combined With Pertussis And Diphtheria?

I would like to know why the tetanus is being combined with pertussis and or diphtheria. Is there any way to get just tetanus vaccine booster, I had at least 4 boosters in my lifetime. just got a bad cut and went to Walgreens ,thinking I could get just tetanus booster there, but they said only had tet,plus dip, or tet plus dip plus pertussis. I was already vaccinated for pert/dipther.? Thanks Old School