You are only ever appealing to get in to school X. You are never appealing to get in to school 'X over school Y'. In other words, you are never appealing against a less favoured school, but only ever for the school which you applied for, and which has rejected you.
As it's a year 3 place, you only have to show that your child would benefit from the place more than the school would be prejudiced by having to take him. So you can look at the school capacity and place numbers, to see if the school is objectively full - are all year groups full? If there are spaces across the year groups, that will weaken the argument the school has about communal spaces being tight for time such as whole school assembly, corridor crowding, etc.
You can also look at classroom sizes - are they big or small? Are they good usable rooms, or do they have silly shapes with unusable areas? (E.g. L-shaped rooms often mean that one area can't be used for whole class teaching. Triangular rooms often have one unusable wall. Some schools will explain that a classroom seems big at xm2, but wall has units along it so no tables can be placed there, and effective space is therefore only ym2.
Look at facilities, clubs, etc.
Go to town - anything you can see that makes this the best school for your DS. But don't say "it's easy to get him there".