When you cast Aid at second level it buffs three creatures max and current hp by 5 for 8 hours. My question is could this stack; either by casting it multiple times or having several spell casters cast it on the same creature?
The effects of the same spell cast multiple times don't combine.... For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once. (PHB p.205, "Combining Magical Effects")
There are interesting cases, also covered in that PHB section, that arise when we consider durations and potency. But you don't mention those in your question, so I don't address them here.