The number of possible tripeptides formed involving alanine (ala), glycine (gly) and valine (val), where no amino acid has been used more than once is:

For tripeptides with 3 different amino acids (ala, gly, val) where no amino acid is used more than once, each tripeptide must contain all three amino acids exactly once. The number of distinct tripeptides is the number of permutations of 3 amino acids = 3! = 6. These are: ala-gly-val, ala-val-gly, gly-ala-val, gly-val-ala, val-ala-gly, val-gly-ala.