Health Boost III finishes the Combat tree's health line with a third added segment. Take it once you reach level 3 if gang wars and strongholds are wearing you down.
Health Boost III sits at position 7 of 18 in the Combat tree. Unlocking Health Boost III takes 2 skill points. You need level 3 and Health Boost II before you can buy it. Jacob and Evie each have their own skill tree, so you buy Health Boost III once for each twin.