If I am not mistaken, it is cheaper to buy a (3-lifetime session key / 3) than it is to buy (2 * 1-lifetime session keys) / 2... without only a small difference in price over-all.
What I am saying is that for the price of the 3-lifetime session, you are only paying just a little more for the 1 extra key. Might as well get three!
They may merge your 1-lifetime with another key, if you purchase another copy, and convert it to 3-lifetime -> but make sure you ask support prior!