Who guard a prize with one another

I think you mean the second line.

I think it means that the prize is guarded by all three brothers together.

Yes, line, of course.
Here's more facts. There are three brothers, three tombs, and three keys. The keys open up the forth tomb with some treasure.
So, do you think the who refers to all three brothers, and the prize it is the treasure in the forth tomb? If so, then how do the one another expresses mutual action?

PS: My though was that one of three bro

Dark Furyhow do the one another express mutual action?

It's not mutual action in this case; it's concerted action.

with one another ~ together

who = three brothers

CJ

Edit: Changed 'each other' to 'one another'. CJ

So, GPY was right.
I had never encountered each other expressing concerted action. Can you give me some similar examples?

Dark FurySo, GPY was right. I had never encountered each other expressing concerted action. Can you give me some similar examples?

Somehow "one another" seems to have changed to "each other", but, whichever it is, it's the "with" that makes it concerted action: "with one another". No doubt this phrasing was chosen primarily for the rhyme.

Still don't get it. Tell me if I'm wrong. Isn't this a folded version of something larger? As in "With Peter working in Chicago, and Lucy travelling most of the week, the house seems pretty empty.

I don't see it as fundamentally different from "I went to the shops with my mother", i.e. signifying that one person accompanies another. For me there is a faint doubt whether the reciprocal or symmetrical nature of "one another" is exactly logical with "with", but I think we can allow poetic licence.

Thank you, GPY, CalifJim. You've really helped me.