Jesus in the Garden 2 Sorrow

It is impossible for us to understand the full extent of Jesus' sorrow and grief in the Garden of Gethsemane on the night He was betrayed.  The One without sin was about to die for the sin of the whole world, past, present, and future.  Jesus came to pray and asked three of His disciples to share in those prayers.  Why did Jesus choose those three, in particular?  What would have happened differently if they had not failed in what Jesus asked them to do that night?