ok in general:
/dev/sd<XXX> is a scsi disk (or sata , fiberchannel, iScsi, flash stick) they all behave like a scsi disk.
/dev/sda is the first disk
/dev/sdb is the 2nd....
if there is a partition table on a disk then the partitions 1-4 are primary partitions, 5-8 are the extended partitions.
YOU need to known on which disk is WHAT.
Then you can take a not on which UUID belongs to which disk.
based on that you can assign the UUID's in a fstab to get it mounted correctly.
There can be another way, you can assign labels to ext2,3,4 disks those labels will be used when the disk is automounted, as the name of a directory.
My advise: the current disk (active OS) is in / (say it is the Gentoo disk)
on that disk make an Ubuntu directory, and CentOS on on the root directory of the disk.
Then mount based on UUID the root partitions of those disks on the resp. /Ubuntu /CentOS etc.
Then mount sub partitions as needed on those disks as subdirectories on then ...
(say /boot of Ubuntu -> /Ubuntu/Boot)
That should make them easily distingishable. You need to take care of the level in fstab (pass column).
That should be 0 for /, one for everything below / (f.e. UUID entry for Ubuntu's /) on /Ubuntu during pass1, and /Ubuntu/boot as pass 2... etc
