I'm a single adopter of 3 (but I adopted them one by one not all at once!)
I don't personally think that social services would like the idea of a single adopter adopting 3 at once. Going from 0-3 children at once is incredibly overwhelming at the best of times, as a singly it would be even harder. I don't personally think that most agencies would be prepared to take the risk, and most sibling groups of 3 are advertised as needing 2 parents by their placing authority
Some single adopters do adopt sibling groups of two though - if that was what you really adopted and social services thought you would cope with that, then that is very achievable
You need to be planning to take 6-12 months off work when your child/children is/are placed
After that, it's fine to have "work full time" as Plan A. You may want to think about what might happen if your child isn't coping with that, and try come up with a Plan B. Most single adopters do work though, many full time. I couldn't work for many years, but I now work part time
It's fine to have your children attending your school as Plan A as well, with the other primary school as Plan B. I would make a point of telling social services that you will make your final choice based on your childs needs. It being a private school is not a problem as long as (and obviously yours isn't) it wasn't a super selective or similar - in that case social services would have said that your expectations are too high I think. But you work in a school that can accomodate children who have some additional needs, so it doesn't matter that that happens to be a private school IMO.
Your housing situation is an issue if you could be told to be moved at any time. Renting is not an issue, it's absolutely fine to rent, but it needs to be more than a stop gap house, it needs to be more long term.
Given that the process is NOT 5 years long (as short as 10 ish months maybe, but it could well be 18 months+), I would suggest that you move into your planned 2 bed house and get settled there first before making enquiries
If you are in a 2 bed house, then you are restricted to sibling groups who are both the same gender, opposite gender siblings can't share. Social services may want you to consider what happens if they can't cope with sharing a room as they get older.
The disruption rate is 2% before finalisation (children have to live with you for some time before it can be made legal in court), and there are no accurate statistics kept about what happens after that. I suspect that if you take ito account the teenage years and teens who have to return to care, the disruption rate is higher - I would probably guess 20% I suppose but I wish we had more to go on than pure guesswork based on our experiences. The majority of adoptions do work out and do not disrupt though
It's fine to say that you will not consider children with serious needs. It depends on age of child you adopt. With a baby, there is more uncertainty and you have much less idea of what their needs in the future will be. You need to have an open mind about certain additional needs and more minor needs, but when it comes to thing like moderate-severe physical disabilities, and already identified significant emotional/behavioural needs etc, it's okay to say 'no'. Social services would be concerned about someone who wanted to say yes to any need, they would think they weren't realistic or seriously thinking about what life would be like with some of these needs.
Hope that helps 