Hi Natasha,
The no common amino acids is a prerequisite when you have the three tripeptides because if they were as such:
1. AAA
2. AAA
3. AAA
with these common amino acids, it will decrease the number of options available to the full sequence, in this case, reducing your number of possibilities for the full length peptide structure to one.
When you stipulate no common amino acids to each tripeptide, you simplify this to a problem of arranging three things, which you can express quickly using factorials. I have outlined a brief approach here, accompanied by a slightly longer approach.
https://alumniubcca-my.s...ye9s6rjeLjeMBA?e=h7pGHA
I hope this helps!
Katt