Ok so it has got a bit complicated (or at least it is about to be) we are looking for clarification on questionmarked parts..
Say A casts magic jar, and possesses B. they have possession of Bs body. C comes along and casts magic jar, and possesses B as well. now we already figured that it is A that is making the will save here, since they are in Bs body, so thats fine. we also figure that A now gets stuck in Cs magic jar, C is in Bs body, B is in As magic jar, and A and Cs bodies are void. (side note, can C, when in his magic jar, detect Bs soul in As magic jar if present?)
C at this stage is really the only active soul as the others are essentially imprisoned.
if As casting of magic jar ends first then B pops out of As jar but has nowhere to go as its body is possessed by C, so dies? C is fine in the body of B. A is still trapped in Cs jar?
if Cs spell ends first then presumably A goes back into Bs body? rather than B the true owner, as they are still imprisoned in As Jar. but if C has taken his own magic jar and made sure it is out of range of As body, then ends the spell: C goes back to Cs body, A leaves Cs Jar and dies as out of range of his own body. His spell then ends so B leaves As Jar and goes back to it's own body as C has now left this.. or are all movements simultaneous and 'blocking'?
if C has put his Jar well away from the scene (as he doesnt know where As body actually is) but it turns out that A was incorporal, does A reform by Cs jar if not in range of Bs body? does this 'force' an end to As casting of Magic jar?
head -> explode
if As casting of magic jar ends first then B pops out of As jar but has nowhere to go as its body is possessed by C, so dies? C is fine in the body of B. A is still trapped in Cs jar?
That makes the most sense, but as written... nothing actually happens. At all.
Magic Jar is a cool spell, but it's in desperate need of a rewrite.