From searching TAC cases, it appears you need to order the upgrade part number that brings you to the final number you want.
In your case, you've mentioned 150 + 50 + 100 ---> obviously results to 300. If there is not a 300 user license on your platform, then you will need to go with the next higher number available.
Do you have specifics like platform and user count that we can give you the exact part numbers you should research further?
thxs
peter