When it comes to tin size, give or take 0.5cm is fine. The best ranges of smaller round tins are at thevanillavalley.co.uk or squires-shop.com. For square cakes, I’m devoted to my Silverwood ...
You’ll be mixing the batter for the three cake layers separately, and baking them for different lengths of time, to account for the differing cake tin sizes you’ll be using. You will need one 15cm/6in ...