Two broodies co-brooding sometimes works, but I've never heard of a successful threesome.
I have heard of a threesome that worked (mentioned in the book "Chickens in Your Backyard.") Of course, being successful once doesn't prove anything about what will happen with different hens another time.
I have 3 hens all gone broody in the last week a few days apart the problem I have is they are all laying in the one nesting box .
what should I do ?
You could let them try it. When the chicks hatch, watch to see how it goes--you might need to separate the hens at that time and either divide the chicks among them or give all the chicks to just one.
You could move two of them to separate nests now. They will try to go back, but if each new nest is in a separate pen, or even in a separate dog crate, you can shut them in. They will either decide to set on the new nest within a few days, or will quit being broody within a few days. If they stay broody on the new nests, you can give them each a clutch of eggs (if you want lots of chicks.)
You could do as Perris suggested, and break the broodiness of two of them.
Common advice for breaking broody hens: put the hen in a wire cage or crate, supported on blocks or hanging from the ceiling. This is to provide lots of ventilation on all sides, but especially underneath her. Give her all she wants of water and her usual food, and you can also give her a roost to perch on--but no nesting material, and no solid floor. You can just leave her there for 3 days and nights and then let her out, or you can let her out for a bit each day and watch her to see if she's done being broody. If she tries to sit on the nest, she's still broody. If she acts like a normal chicken (eat, drink, dustbathe, sleep on the roost at night) then leave her out of the cage. But check again after dark, because if she's sleeping in the nest again--back in the cage she goes!
