One way would be to install the OS on one PC, set the system up as you want, and then use Clonezilla
to create a disk image which can then be installed on the other PCs. I think this would depend though, on the machines being very similar hardware-wise.
Edited to add
Although I think I may have misunderstood. I was thinking "install the same distro on several machines", rather than "install different distros on the same machine".
How about copying the /etc/password and /etc/shadow files from one distro to the other? There may be a problem if the distros use different hashing algorithms for the passwords, but otherwise it should work. Not sure about the UUID.
The other thing to watch would be if you were sharing home directories. There may be a conflict over the hidden "dot config" files, with the distros expecting different setups in the same files.