I made the mistake of not wiring all 3 lights together, only two above the stairwell going to the basement and then a separate light in the basement. I'm hoping there is an easy wiring path to allow me to add the basement light at the end of run.
If I understand you correctly you want to have 3 switches control several lights which land separately at the start and end of the switch run.
You can do that by using a 14/4 (if on 15A breaker, 12/4 if on 20A) to carry the neutral, travelers forward and switched live back.
That way you have a switched live at every location.
The other option is to split the light cluster and control them separately, but I'd need more detail to suggest a plan to do so.