Mickj, back again with more time :)
The frequency with which you're feeding means you are probably only feding 6-7 times in 24 hours. For many women, this isn't enough at the beginning to establish and maintain a good milk supply, and the cracked nipples makes me think the baby was not positioned in a way to feed effectively, too.
The best way to improve weight gain for anyone - not just babies! - is to eat often. With breastfeeding, this also stimulates a good supply. Milk supply is driven by the frequency of effective feeds.
Ask your midwife about feeding Finn more often - lots of skin to skin contact, co-bathing, co-sleeping, not waiting until he cries to feed, and responding to feeding cues promptly will almost certainly increase his 'visits' to the breast. If she feels he still needs supplementing, discuss reducing the freq and amount of supps. 60 mls at every feed is a lot, believe me, and it is likely to be reducing his feeds.
It is not normal for him to be still losing weight at 3 weeks. An initial weight loss is normal, but babies should not continue to lose after the first days (though they may gain slowly). You'd need to be looking for frequent, soft yellow poo after about day 5 as a sign all is well - did this/does this happen?
I'd also wonder about how he was weighed. If he was naked on electronic scales then you can have some confidence in the reading. If the midwife used 'hammock' scales or if the baby was in a nappy, then there is no way anyone should make a clinical decision based on that reading.
I hope you'll post again, with more info....and I really hope you can find someone to listen to you properly, and who understands how difficult and stressful it can be to be in your situation :(