I like your analysis and have similar thoughts but some differences. I would like to first mention that I am more familiar with Bane and Kun than Krayt so take my thoughts with that in mind.
While it is true that Bane's armor has a weakness to lightning, Krayt's does as well as that is how he was killed the first time by Wyyrlok. Bane was also very strong in lightning so I feel the armor/lightning combo between the two is dead even and in this case would come down to whoever would get a lucky sneak attack lightning strike in.
As the prompt states that the 3 do not know each other this gives Kun an advantage. The only thing Bane feared was Dark Magic and Kun was one of the best practitioners of all time. If given time to gather his power while defending attacks (as Zannah was able to do in the final duel vs Bane) with the boost from his amulet, I think Kun would win. (If they did know each other, I think Bane would target Kun from the start to rid himself of this threat).
However, and this I feel is the more likely outcome as I don't think Kun would get the aforementioned time, if the fight is strictly upon lightsaber skills, Bane will win. While all 3 are fantastic lightsaber fighters possibly among the top 10 all time from either side of the force, I consider Bane with orbalisk armor to be in the top 3 duelists of all time with Palpatine and Luke. With his orbalisk armor, Bane was able to hold his own and at times overwhelm a duel wielding Jedi Master considered to be one of the best Jedi fighters of all time, another Master, and Knight of whom the duel wielder was using his scrub attacks to her advantage. And, all three of these combatants were fueled by a 3rd Master whose only battle skill was meditation. Given that Bane performed such a feat, I don't believe that any combination of effort of Kun and Krayt against him would be quite enough to defeat him and thus I see Bane winning.
Take note that as I stated I am more familiar with Bane and Kun I see two scenarios with one of them winning and none with Krayt winning.