There's not really "catchment" as such. It varies year to year.
Generally, admission policy is :
Children in care first
Then siblings
Then everyone else, based on home-to-school distance, so closest first, until all places filled.
Faith schools will have a category for baptised children too.
So "catchment" can vary from year to year, depending on demand. If it's a popular school and a high sibling year, a non sibling might need to live within 100m to get a place. The following year it might be 900m. Most boroughs publish a document giving previous years figures.
Bear in mind schools can go up as well as down. When I moved to this area, school a was outstanding, school b so bad it was about to be demolished. When it came to dc going to school, school a was on its way down to satisfactory, school b had been rebuilt, lots of money thrown at it, and in very high demand.
I'd find an area you like and can afford, worry about schools when your dc is nearer 3.