Unsure if my solution is right. Might need a mod to confirm.
So you have 3 posters and you need at least 1 of each.
Let x, y, and z be the 3 different posters.
Since you need at least 1 of each poster, you have 7 ways to pick x, 6 ways to pick y, and 5 ways to pick z.
Use the Binomial Theorem and you get \(\frac{7!}{3!(7-3)!}+\frac{6!}{3!(6-3)}+\frac{5!}{3!(5-3)!}=65\).
I'm not sure if I did this problem right ;-;