Lexeme refers to a single word and all of its forms. For example the word "go" in English has the forms "go" "goes" "went" and "going". All of these words are from the same lexeme "go." Morpheme refers to the smallest unit of meaning a word can be broken down into. For example the word "cats" This can be broken down into "cat-s" "cat" carries the meaning of the furry four legged animal and "-s" carries the meaning of plural. Sememe I am less familiar with, but it means the smallest unit of meaning. It usually means the meaning that a morpheme has. It is related to semantics. So.. if you had to break it down.. a lexeme is made up of morphemes and morphemes have sememes. I hope that helps.