أُناس : جمع إنْسِيُّ
لبَشَرُ : الإنسان ( الواحد والجمع والمذكر والمؤنث فيه سواء ) وقد يُثَنَّى ، ويُجمع على أبْشَار
أشخاص جمع شخص = a person
so the first one means humans beings , but it can be used as "persons" as well , not in the common vocabulary , you find that more in literrary style or in classical Arabic as Westrens like to call it !
the second one is also human but it's general it's the accurate tranbslation of human and it can't be used in place of the first and the third
the forth one is more common to refer to persons ..the first is less common !
I hope that answers your question !
and Thanks again for this smart question !