If you, or the HV, think your baby needs extra calories (and that's all a baby of this age would need,as specific other nutrients are not needed at this age) then giving extra breastmilk is the most effective and appropriate of delivering them.
There is no point in giving solids at this age to increase calorie intake - it won't work, and in fact it could lead to fewer calories being used.
If you give veg or fruit, the calorie content of the tiny volumes a baby of this age would have is very low....but the baby might be too 'full' to take as much breastmilk.
Even if you give comparatively calorie dense food like rice, you still have to give a lot of it to make any difference. Read the packets - I think a teaspoon of reconstituted (with water) baby rice is something like 8 calories. How many teaspoons is a 3 mth old baby likely to have - five? six? so maybe 50 calories, tops? She'll use up calories digesting it, too! She might as well have a breastfeed instead, and take (probably) 70 or 80 calories, (for a 3 fl oz breastfeed, typically) metabolising it easily and economically.
The quality of your milk is just fine. Milk quality varies hardly at all, and certainly not significantly enough to make a difference to a baby's weight.
It sounds as though your baby is healthy, but if there is a question mark over her weight, then just offer her more breastfeeds!
Messing about with titchy bits of puree and slop is a chore....easier surely just to stick her on the breast