So you won your appeal, now you're looking to challenge them again if they amend the policy because it would be 'unfair' to your family? You wouldn't stand a chance I'm afraid.
It's a school's prerogative to change their policy at any time they like, as long as the way they do it is above board. You won your appeal because the policy wasn't clear. If they make it clear, they'd be doing nothing wrong if your youngest child doesn't get in in the future. To be honest, your appeal is probably what has prompted them to amend the wording of their policy. I think you're realised this and are now panicking, right? You were thinking short term about child number 2 when you pushed for an appeal, without considering the impact on that in the future for child 3. They realised they risked people appealing every year under the current woolly policy, so think it's better to change it. Their prerogative.
That's the risk you take by applying to a school over 2 miles away. Even if one child gets in, there are no guarantees for future siblings if the policy doesn't prioritise them (even if a previous one did). It doesn't matter how long-standing the previous policy was, policies are always subject to change.
Yes this would be a pain for your family, but changes to admission policy will always be a pain to someone. What benefits some under certain policy changes, creates a headache for others. That's the nature of these things. You can never please everyone. On the flipside to your dilemma, someone else more local to the school than you could argue it's unfair to their family if their eldest child can't get into what might be their most local school because places are all taken up by families who live further away, under the current sibling rule.
Most school admission policies give priority to looked after children and children with an EHCP naming the school as the most appropriate for that child, then siblings in catchment, then other children in catchment, then siblings out of catchment, then other children out of catchment. It sounds like your school want to bring their policy in line with this, to make it fairer on local families rather than prioritising siblings regardless of how far away they live.
Basically, you were quite lucky your eldest got in to a school 2.8 miles away, you were very lucky the policy was so woolly that you won an appeal for your second but it sounds like your luck might be about to run out for your third. Sorry.
Schools absolutely should prioritise local children. Siblings within the immediate locality should rightly be given preference, but out of catchment is a major risk to apply, if you have more than one child, as children within the catchment area should come first. For the local families who have previously been missing out, this change sounds very positive.
As for this not being 'fair' on your family, welcome to life. It often seems unfair. However, the school are actually trying to do what is fair for the families in the community they serve. As an 'outsider', your family shouldn't be a priority above those families. if this policy change is passed, after following the correct procedure, there's bugger all you can do I'm afraid.