Ok so here's the deal.
My brother told me that you can fuse all three of of these legendary Pokemon together. My instant response was "Yeah Right" because then you would somehow have to balance the four types of Pokemon in one (Dragon/Electric/Fire/Ice).
Of course it could just be a dragon Pokemon with the legendaries signature moves.
Anyhow, Is it possible?
You can fuse Kyurem with zekrom creating Kyurem-B and kyurem with reshiram to create kyurem-W but only in B2/W2 with the DNA splicer. Kyurem-B and Kyurem-W both keep the dragon/ice typing. You cant fuse all three of them together or zekrom and reshiram together. So you brother is sorta correct but mostly wrong.
You can fuse Kyruem and Zekrom together to make Black Kyruem.
You can fuse Kyruem and Reshiram together to make White Kyruem.
Your brother is wrong. You can't fuse Zekrom, Reshiram and Kyruem together.
And you fuse them together with the DNA Splicers.