Fremennik sea boots 3 are footwear obtained by completing the hard Fremennik achievements and speaking to Advisor Ghrim on Miscellania to claim the rewards. You must have obtained the Fremennik sea boots 1 and 2 first (so you must have completed the easy and medium achievements as well). They can be reclaimed from him if lost. It provides bonuses in-between mithril and adamant boots.