An easy way to work it out is to fill up whichever tin you have a recipe for (eg if the recipe you're following is for an 8 inch tin) about halfway with water. Then pour this into the other tins so they fill up about halfway too and that will give you an idea about how much to reduce or increase your ingredients by. However, I'd personally google a cake conversion table and work from that - it will tell you how much to multiply or divide your recipe by for different shapes and sizes of tins.
Regarding the layers, I'd say make at least 2 cakes in each size. Most standard cake recipes will give you a cake that's a couple of inches tall, maybe 3 max. If you only use one cake per layer, even with a filling, you'll only have a cake that's 8 or 9 inches tall which really doesn't make an impact when it's tiered. Either cut each cake in half so you have 4 thinner layers per tier or just tort each cake and have 2 thicker layers per tier.
If you need a way to visualise it, stack your cake tins on top of each other and then take away half that height- that will be with one cake per tier - assuming your tins aren't extra deep ones. If you feel it needs to be taller still either make the tiers taller or make a 4th tier - this would be a more practical solution when it comes to cutting and serving the cake but you may need to invest in another cake tin!
