Who are you doing it through? I'm working on a 3 peaks in September and it would be a co-incidence if it were your group.
At the end of the day you've done the preparation that you have done. It's too late to do much about it now, so you are best to stop worrying over it and just crack on. Yes the 3 peaks is a tough challenge, but there are worse ones out there. The 3 peaks is as much psycological, dealing with the lack of sleep, and being on the go for so long, as it is physical. A good steady pace uphill is fine.
One of the keys to success is cutting out the faff. Have agreed rest stops, say 5 mins per hour. But make sure that you stick to this. It's not stop, take your sack off, rummage for some food and drink, sit having a bite to eat for mins, then start packing your stuff away. It's 5 mins from stopping walking to starting again. Also make these your only stops if possible. If one person stops to take a layer off, everyone does it then or waits until next stop. It's amazing how much time this kind of organisation can save. If you stop to take a jacket off, then it will take a couple of mins to stop, take it off, put it in your bag and get moving again. So if everyone does it at different times that's 2 mins times the number in the group, possibly several times per hill. It could easily add up to half an hour or more over the entire event. Another faff point is getting into and out of the vehicles. You've got 5 of these to do (out then in at the Ben, out then in at Scafell Pike and out at Snowdon). So take 10 mins at each of these and that's 50mins. You want to get boots on and bags ready as you approach the car park, ready to be straight out and walking as soon as you stop. Likewise as soon as you are down, straight onto the bus and take your boots off, have food, sort things for the next hill while you are moving. So those faff savings could be saving you and hour and a half or more, which translates into either a faster time or, more importantly for you, more time for the walking and less pressure on you.
Make sure you eat plenty. You could be using 10,000 calories over the even and you need to replace that. Keep easy to eat snacks handy while you are walking, cereal bars, flapjacks that kind of thing. Not just chocolate bars and sweeties, because these release their energy fast and are used up fast. However they are great for moral, and give a useful energy boost when you are flagging on the last stretch to the bus, so worth including, just don't rely solely on these. Then something nice and substantial for while you are resting in the vehicle between hills. Sandwiches, pasta salads, my favourite is pork pie or something like that. Just make sure it is something you really like. In the middle of the night you're body doesn't want to eat, but you need to. Everyone always gets down from Scafell Pike and turns their noses up at their food and you see them really struggling to eat it. So keep your favourite for this meal.
If you're finding it tough after the Ben, then don't be afraid to sit out Scafell Pike and join back in for Snowdon. Scafell Pike is the least nice of the three (especially as it's at night) and everyone will be a bit more tired by Snowdon, so after the extra rest you will find it easier to keep with them.
A pair of earplugs are useful for in the bus (and maybe a mask too) to help you sleep.
The main thing is don't stress about imagined worries, take it as it comes and try to enjoy it.