I completed the Saint Georges River Raids back when it first released, but now when I look at the River Severn status, it reads "Gear and Weapons 2/3".
After doing a little bit of research, I've found that I have the fives pieces of armor, along with the sword and shield. Did Ubisoft add another item to that river, or this is another one of their glitches/bugs?
Not sure if this could be the reason, but I am playing the PS4 game on the PS5.
@spirantcrayon22 Thanks for providing this information to the thread. @reyrod01 please let us know if you are able to get to 3/3 after going back to the earlier river raids.
Hi vikings!
Sorry to hear you're also struggling to find the last piece of loot @clarke83ash ! And thanks for help @azullFR and @bielik01 !
Have you managed to find the missing piece after completing all the River Raid quests and defeating the champions? As it was mentioned previously, some of the loot was added to old raids in updates to the game, so it's worth to go back there after finishing the new ones! 
If you are still struggling to find them or have any additional questions, please let us know.
devtrackers.gg