1) UMS' - don't like this. It's not a singular noun that ends in s (e.g. Wales, Moses, Jones), so we can rule this out.
2) UMS'S - nah, the capital S is not required. It doesn't represent another word (i.e. it's not an initialism). This option is only feasible in a title where you have to use caps.
3) UMS's - hooray. This is the badger.