Is it possible that there could be more than two sexes in an alien species

I know it is possible to have 1 or 2 sexes, but is it possible to have three types of sexes. If so could you explain to me how reproduction would work? This is a serious question.

Yes it could be perfectly possible. Many things could be possible when it comes to life on other planets that we would find very strange. For example all life on Earth is carbon based, but there's nothing to say that on another planet it couldn't be nitrogen or oxygen, or even something bizarre like chlorine or mercury based.....

As far as how it could work: humans are diploid organisms, meaning we have 2 full sets of chromosomes; 2 copies of each one. However, there are some species even here that can be triploid or tetraploid organisms. Usually it isn't natural and the organism cannot reproduce or may not be able to survive, but sometimes, especially in plants, they can have desirable traits. For example, grass carp are common in some ponds to keep it clean of weed