Prove using the method of double counting: $3^n =\sum_{k=0}^n \dbinom{n}{k} \sum_{j=0}^k \dbinom{k}{j}$

Question

I encountered the problem: $$3^n =\sum_{k=0}^n \dbinom{n}{k} \sum_{j=0}^k \dbinom{k}{j}$$

which shall be proofen using the method of double counting. Since I'm not that good with this method, I'm thankfull for any help. Unfortunatelly I haven't found any hint on that.