The problem does not make much sense to me.
Suppose the first 15 out of 25 attend session one.
Suppose the 10 other plus 8 of the first 15 attend session two (so that 7 of the ones that attended the first session do not attend the second).
Then you could have that the 10 other again and two of the 7 attend the third. That way, no one attends all three.
The answer would be zero.