Usually, if the bees are dying right near the entrance. They are shaking, walking funny and spinning on the ground. To me it is a pesticide poisoning. I have tried 2 things on the past. (1) is to close the hive off with screen and let it sit for 48 hours. But this is best done with a screened bottom board or in a shaded area. After it has set I opened the hive to remove the dead bees.
(2) is to let it take it course.
I find that when I close the hive down I have a better success rate of some of the hive surviving.
Bees will only go to one source and they keep on that source until it is depleted. That is why they other two hives at this time are OK. Hopefully they don't find the source in the future.
Please realize will be tough to determine how far the hive will crash. Depends on the pesticide strength. I have had some totally crash and I have had some go about 1/2 down.
If you have a partial hive that makes it then there will be a good chance you will lose your queen so check it again in 7-10 days. You will have to replenish the hive with a frame of brood with eggs so they can make queen cells. Then if possible a week later give then another frame with brood with eggs. That should help.
Could you post some pictures?