Obviously asking what people spend, is a bit of a 'how long is a piece of string' question - you can spend a fortune if you choose to, but I'm presuming you are asking as you are worried about your budget, so, to follow your thoughts:
When my (18yr old) ds left for university, my food bill went down around about £15 per week, so multiplied by 3 boys and multiplied by 4.5 weeks per month that's closer to £200 than the £450 you predict. Yes, you can get £££ worth of cakes and biscuits which they will hoover up, but there's ways of filling them up much cheaper than that.
I'm not sure what you are planning to spend £180 (£60 per ds) a month on , on clothes and equipment. Yes, replacing shoes is expensive when they have a growth spurt (although things like their walking boots and wet shoes can be handed down), but most months you don't need to go out and shop for new clothes. I don't spend anything like that on my 3 dc.
Not sure what you mean by equipment as you say they don't do much. Mine get big things - like a decent rucksack for Scout camp - for birthdays / Christmas
Phones - with so much wiFi readily available, my dc just use that. They have PAYG phones (so just the initial purchase, £30 or so) and they fund any top ups after that. They use WhatsApp mainly, or facebook Messenger so it doesn't use up any of their texts, so £20 credit lasts them about 18months - 2 years.
Pocket money - that's a whole other question which produces a wide variety of answers, but mine get £1per month, per yr of age, so my 16 yr old gets £16per month from us, and goes out to work to earn her big money.
Your electricity probably will go up, but I'd have thought your heating wouldn't change much. Difficult to guess that one.
Scouts - my Scout pays £10 per month for 11 months of year and my Explorer pays £2 per week, so for 3 of them it should only be £30ish a month. Admitedly it will start to add up as they go on camp, so your estimate might be close across the year, if they go on a lot of camps. If they don't (or the group lets them fundraise?) then you've over estimated there.
I doubt you'll spend anything like that on trips out (unless you are budgeting for a ski trip or something). Secondary schools don't do that many trips you need to pay for, and, 'as a family' teenagers don't tend to "do" trips with their parents 
So, the good news is, IMO, it won't cost as much as you think in money, we won't go into stress levels.