Based on what you've said, I think perhaps if you do tell her it's the only way it'll work she'll likely back down and say ok fine then.
I would, however, consider whether it's worth using it as an opportunity to opt out as others have said.
Your friendship is going to change massively. Your priorities will change. You will change. Is she prepared for how different things will be once you have this baby as it doesn't sound like it.
Is she aware of how often you may have to leave to breastfeed? Is she expecting you to be able to do it to a perfect routine? To delay and wait if it's not a convenient time? What happens if baby needs feeding during the ceremony? Is she expecting you to leave baby screaming if it's photograph time? How long for? Is she ok with you quietly nipping out repeatedly or does she expect you to let her know each time?
Baby may still need feeding a lot and you going up and down is going to be a lot more disruptive than a baby in arms would be so it's an odd decision from her especially as you're a bridesmaid and presumably your repeated absence is going to be felt.
You could potentially still only have a 10 week old and I cannot emphasise enough how hard her fourth trimester can be and how much I was still in survival mode.
I can barely remember it and having to go abroad to a wedding and then leave my baby in a hotel room would have just not been something I was willing to do at all.
I was absolutely not the stereotypical anxious modern mum who doesn't want people visiting or holding the baby. I loved having people around him and holding him just as long as I was there too.
It's hard to describe how much more in tune I was with him than others, how much more comfort he found in me and the overwhelming urge I had to stay near and to scoop him up when he cried.
I remember finding it really distressing if he cried when being held by someone else if they didn't give him back quickly enough or if someone was helping make up a feed and not doing it with enough urgency as he cried. I knew I was being insane and expressed nothing but gratitude to the people around me but it was an overwhelming feeling.
I remember around the same time lying in bed and thinking completely seriously that if I could 100% guarantee that someone would love him as much as I did, I'd give him away to them in an instant. I did not go on to have any postnatal mental health issues at all. It's just those first weeks were absolutely horrific. I was recovering from a traumatic birth, was so sleep deprived as he had silent reflux and colic, and was living life in a haze.
Admittedly by 3 months things were definitely improving and I was becoming a lot more sane, so perhaps I would have been on the better side of it, but going abroad is pretty massive at that time and you'll potentially still be recovering.
Everyone is different of course but it is worth really thinking about it and ensuring if you do go that your friend understands how young 3 months actually is and what that is going to look like.
People have mentioned sharing the room with your mum, which I guess if you're not going with your partner could potentially work but I'd also consider that your mum is probably going to find the day really exhausting and ideally she deserves at least a good nights sleep in her own room.