The number of tripeptides formed by three different amino acids...

A tripeptide is a peptide consisting of three amino acids joined by peptide bonds. The exact nature and function of proteins is determined by the amino acids present and the order they occur.

 In these, an amine group from one residue forms an amide bond with the carboxyl group of a second residue, the amino group of the second forms an amide bond with the carboxyl group of the third. 

Let $3$ different amino acid are $A, B, C$ then following combination of tripeptides can be formed-

$ABC, ACB, BAC, BCA, CAB, CBA$