Hmm, I was going to complain about the arm not being explained, but if Oda intends to reveal it later then I guess he didn't want to reveal it now. Must be related to the SMILE stuff.
Anyway, the first episode was really good, the second episode was alright, and this third episode was honestly pretty meh. Not nearly as bad as the likes of Little East Blue or Silver Mine, mind you, but still sad to see potential once again screwed up in fillers. I liked these three rookies and I don't think they should have punked Luffy by any means, but it would have been nice if they could have lasted more than a few seconds. That vice admiral was a joke as well.
They probably should have tried to expand this for another 3 episodes or so by expanding the fights a bit with Nami, Chopper, and Carrot. And maybe had Brook and Pedro get in on the action a bit. I doubt this will be much of a buffer to help the pacing in Whole Cake Island.