Any suggestions for a caulk-like substance to be used for 3 tier birdbath stabalization?

I recently bought a multi-tier bird bath made of concrete for my backyard.
I was certain (100%) that the base and main dish were level. What I found was the other tier sections have unlevel bottoms.
I am looking for stabalizing material – like caulk – to both level the sections and stabalize them if I [...]