Fusion is a new way to combine Lieutenants. Unlike the normal combine feature, fusion fuses any three different lieutenants of the same rarity into a single stronger one. To fuse lieutenants, first select the lieutenant you want to upgrade. Then select two lieutenants of the same rarity and star power to fuse into the first one. Additionally, you will need a recombinator of the same rarity as the lieutenants to perform the fusion process. Recombinators can be found in challenge jobs.
Note: The fusion process cannot be reversed, so make sure you don't fuse away a lieutenant you want to keep!