Dean: America's Sweetheart By Elle King

This was the song that had you and Dean meet and you both were thankful for that song ever since.

Sam: Don't Let Me Down By The Chainsmokers ft. Daya

You and Sam had made a pact that you would both be there for each other and not to let each other down.

Castiel: Stand By You By Rachel Platten

The song speaks for itself. You both are going to stand by each other through Heaven and Hell.