Is the school its own admissions authority? (Faith school, academy?)
If yes, I think how admissions are handled in practice depends on the individual school.
Our school for instance would never let a new child start in the last week(s) of the school year, or even the term. We have had a number of children leave the school, e.g. going abroad, and leaving 2 weeks before end of school year. With long notice.
Despite waiting lists, these places were not offered until 2 days before end of school year, with a start date of September. And that was only after pushing - the school had intended to have the admissions committee meet in September, to then offer the places in week 3 or 4, possibly to start after October half-term!
But I know other schools do things more efficiently, and if one child has their last day on a Friday, the Monday might see a new child joining.
What I am trying to say is, for your middle child at least, there is a good chance that if they don't offer your eldest a start date of 'last week of school year' - so DC won't be 'on roll' until September - there is equally a possibility that any spaces for your middle child that come up over the summer won't be offered until your eldest has actually started school in September.
If they are efficient, eldest should be able to start in July, middle one benefits from sibling link.
If they are not efficient, eldest can start in September, but spaces for middle one are not offered until September, so again, benefits from sibling link.
I think it might be different for your youngest though, as it is reception entry with movement before starting being the norm rather than the exception.
The other thing I'd mention is that you should check how exactly the sibling link is worded in the admissions documents. I've seen all sorts:
- a child who has a sibling at the school
- a child who has a sibling who will be on roll at the time of admission
- a child who has a sibling on roll at the time of application
- a child who has a sibling on roll at the time of application, who is expected to still be on roll when the child starts
- a child who has a sibling who attended the school at some point in the last four years
... and all sorts of variations and specifications to the above!
So it's worth being very clear about what does and what does not put you into the sibling category at your school.