Year 3 seems a bit young. When mine where that age I kept on top of their maths skills. Making sure that basics like time tables etc are solid ( games are good, and maths questions can be played walking to school in the mornings or during car journey, it does not have to be a formal pen and paper session.) General maths knowledge like how many days in a year, grams in a kilo, meters in a kilometre, how may cards in a deck, ¼= 25%, etc… There are also good free maths games available online.
Also read, read, read…. My oldest hated reading at that age, we got talking books from the library (that way they will still pick up the vocab) talk about what they are reading, ask questions to check their understanding such as Tommy went out in a Tshirt, so which season do you think it is… It may seem simple but questions like that make them think about answers that can perhaps not be copied from the original text. Find manageable articles in papers (nothing to heavy)…
I think the trick is making sure there are no (or not too many) gaps to fill when the time comes to start preparing for the test.
In terms of catchment areas there is no certainty that they will not change in the future but a good guideline is the admission brochure which you can also find on the Bexley council website. Hope the link works.http://www.bexley.gov.uk/CHttpHandler.ashx?id=1901&p=0
Finally, relax and enjoy year 3. They are still very little. On the other hand if you are thinking along the lines of getting a tutor (we didn't and I certainly don't think it is necessary to have one) it may be an idea to start asking for recommendations early as sometimes these get booked up early.