Diaphragm: Nerve supply

"3 from 3"
Think: 3 roots from C3 ie., C3, C4 (dominant) and C5. 
These nerve roots unite to form the phrenic nerve which innervates the diaphragm.
Damage to the phrenic nerve will thus lead to diaphragmatic paralysis.

Also,

• C 3,4,5 keeps the diaphragm alive!