I’m the older daughter in a very similar situation. I went to Uni at 18, my sister stayed with my parents and did an apprenticeship. Like you, my parents charged my sister rent and sent me money, for very much the same reason: they wanted to teach us responsibility with money but also wanted to support us.
I don’t think you’re being unreasonable but I do think you need to be careful about how you do this and the message you send. Your son has rent and bills to pay, your daughter is also being told she must pay rent and bills, all fair. However, I think some of what you have said raised red flags for me:
1: your daughter is 17, you have not mentioned anything about charging either of them rent in the past, did you not charge your son rent while he lived with you and went to college? Did he work? If he was allowed to live with you without paying rent until 18, regardless of his working situation, I think if I were either of your other children I would resent being charged rent before then and may well feel punished for not going to college/Uni.
2: you have said you are going to support your son and “hopefully” he will get a job, which suggests that you are putting a near unlimited and unequal financial support system in place for your son, which won’t teach him a work ethic or financial responsibility and could definitely cause resentment
Here is my suggestion:
1: unless you did charge your son rent at 17, I would suggest saying to your daughter and youngest child that they will not be charged rent until the September after they turn 18, this is then equal to what you provided your eldest child.
2: However, your initial discussion with your daughter was very fair, which was that she could do with learning some financial responsibility, etc, so I would suggest to her that she sends you money each month that you save for her, or set up a no withdrawal ISA for her, so that while she’s earning but under 18, she is still learning financial responsibility.
3: Limit your financial support to your son, and make it clear to all your children that this is what they would get when they leave. I would suggest £100 or £200 a month, for example, which should cover the rest of his rent, and expect him to get a job to make up the rest. Uni towns have plenty of places for students to work and students have a lot of time to work - I worked 4 days a week at McDonalds when I was at Uni and it honestly prepared me better for working as an adult than Uni did! This way you are also teaching him financial responsibility and making sure he develops a CV and workplace skills, and setting a clear expectation for your youngest child as to what their future could look like either way.
4: finally, make it clear that when he moves home (summers and after Uni) he will be expected to pay you rent as well. That way, there can be no resentment from your other children and instead you have made clear and fair rules for your household: live with me and pay rent, go to Uni and we will give you some support but you must also support yourself.
As a final thought - I discovered when I turned 30 and was no longer living at home that part of the rent my parents charged me (both my sister and I paid rent from the age of 16, and we both lived at home for some parts of our 20s and paid rent then), went into a savings account for me, so when I buy a house I already have a lump sum. I never knew at the time of course so the financial lessons were well learned! I’m not saying you should do that, but my sister and I felt really supported and it’s unbelievably wonderful to know now that I have some money for that.