it is just for backup purpose single machien carry backup of all machine at the end of the day
I assume you were replying to my comment about allowing root SSH capability. It really doesn't matter what the purpose of the machine is. It is still a bad idea and if you find that this is what is required to make something work it is an indication that something hasn't been setup properly.
If you wish to further restrict access to being from these three machines only, you could also configure the firewall (IPTables) to allow SSH connections from only these three machines.