I'm not sure what to suggest.

As long as he is neutered, they'll not fight to the death. (Sadly intact cats do sometimes)

As he is new, he and your cats will have to decide on a pecking order and a timetable for use of all common paths though the garden etc.

I would be tempted to let them sort it out themselves. Unless there is injuries.

There is little point, in you throwing water etc, that just stops the fighting when you watch. They'll still be at it the moment you are NOT watching.

Best of luck, keep us updated.

PS. Thanks for the compliment from my array of cats