You can split hairs all you want. If there is a difference, do colleges have departments of any two of IS/IR/IA? No. They all deal with the same sorts of things…they pick a name and stick with it. There could be slight differences, but that has more to do with the faculty specialties, not the name of the dept. or major.
At the grad level, you will find some programs that lean towards security, economics, diplomacy, or whatever. You will find some are interdisciplinary & draw faculty from different departments; some that have their own IR/IS/IA dpartment & have their own faculty; sometimes it will be a part of the poli sci dept.; some are in their own college of deplomacy or IA/IS/IR. All arrangements might put a slightly different spin on things, but to claim, for example, someone might get a BA degree in IS and be deemed unsuitable for a grad program in IA is to assume the difference in name has a practical difference.
I just helped my kid examine dozens of grad programs in this field. Some were more practical; some were more theoretical, etc. But nowhere did I see anybody making a big deal about the terminology relations/affairs/studies. It’s like some colleges have depts of Government. Others have Poli Sci. Is there a difference? Do any have both?