Weight gaining

Could anyone give me some advice on how to make my three month old gain quicker? She’s breastfed but our nutritionist says to supplement with a teaspoon of formula for every three ounces of breast milk. Problem is she barely wants a bottle. And my husband thinks i should put more than a teaspoon but I’m scared of constipation. We recently found out she has heart failure and has to get heart surgery soon so the more she can gain the better. Since we found out she’s lost weight. Any advice ?