I am going to be doing a 3 tiered dummy wedding cake for display but I don't know if its necessary to put a board under each tier (as we do on normal cakes). Also, how do I make sure they are all secure and won't topple over due to the lightness?

Stick them together with a little piping gel in the centre of each layer. By the next day the tiers won't move. Cardboard is not needed between the layers..

Or royal icing,
or melted white (preferably white) chocolate.... whatever you have handy in any of these mediums.

I've made them with just buttercream under each tier and it drys hard and holds just fine. No need for boards under each tier.

KimAZ