How many tripeptides can be prepared by linking the amino acids glycine, alanine and phenyl alanine?

Tripeptides are amino acids polymers in which three individual amino acid units, called residues, are linked together by amide bonds. 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. Therefore, glycine $\left(\mathrm{NH}_2-\mathrm{CH}_2-\mathrm{COOH}\right)$,alanine
and phenyl alanine
can be linked in six different ways.