Pumping

I have been exclusively pumping and I only get an ounce per breast give or take and I have had to supplement with formula. Due to an injury I cannot breast feed. I pump every 3 hours for 15 minutes. How can I increase my supply as I wanted to EBF before injuring my shoulder shortly after baby was born. Baby was also losing too much weight so we were told to supplement with formula. My daughter is almost 3 weeks and sometimes will drink 4-6oz of formula in a feeding. She is fed every 3-4 hours or less.