I am doing a square stacked cake (3 tiers) it's all BC what I was wondering is how many pieces of cardboard should I put each tier on to prevent my buttercream from cracking, or should I just use the plywood that I have cut to size?

I would advise using cut plywood or something sturdy under the bottom tier. All other tiers are fine with just one cardboard cake board.

I use 1/2 inch fomecore under the tiers; very sturdy.