I have three geographical locations, all with varying numbers of computers, in total i have 90. I purchased 90 workstation licenses but of course only got a single key. Since i'll have 3 independent admin servers will I be able to apply the key three seperate times or will I have problems and need to get three seperate keys?

You can use the same key in all of your locations.

You must ensure the total amount of your 90 clients.