Instructions
Walk up to Sergeant RL-3. Press "X" to initiate conversation.
Tell Sergeant RL-3 you would like to open his inventory. This will open the inventory screen.
Press "R2" and highlight "Stimpak." Press "X" and select an amount to transfer to Sergeant RL-3.
Move away from any enemies. When not in combat, Sergeant RL-3 will use as many Stimpaks as necessary to repair itself.