I do recommend you speak to the council just to get them to explain their criteria as it has changed in the last two years. I have found their education people to be quite helpful.
My understanding (and I could be wrong) is that B&H will look at your choices in order, so you would only get your 3rd choice if you didn't qualify (ie were too far away from) choices 1 and 2.
However, once Looked After Children/SEN and sibling places have been allocated, the criteria is just on distance, so everyone who has put say Balfour down as either 1, 2 or 3, will be put into the pot, and places allocated on the basis of distance. Thus someone who had put Balfour 3rd might get it above someone who had put it 1st, if the person who put it 3rd didn't get their 1st two choices of school, and lived closer to Balfour than the person who put Balfour first but lived further away.
Does that make sense!
On that basis you would be advised to put down a school you DO like as your 3rd choice, assuming it is not too far away, or as you say, you could be allocated anything...
Roughly wheerabouts are you?
Another way to look at it is - say you put Balfour
Downs
Stanford
in that order, the council would try to allocate you Balfour first. If you lived further away than others who wanted it, then they would put you in the pot for Downs and then see how far away you are compared with other applicants etc.
Right, I am going to shut up now, as I fear I am not helping lol!
Let us know what the council say, won't you?