If I sacrifice Soulfire Grand Master to pay for Collateral Damage, would I still gain the 3 life?

Edited Scott Marshall (Jan. 21, 2015 05:24:52 AM)

No, you would not.

Soulfire Grand Master's lifelink-granting ability only applies while it's on the battlefield, so once you have sacrificed it, Collateral Damage will lose lifelink. Since Collateral Damage will not have lifelink at the time it is resolving and dealing damage, you will not gain life.