Noob Help, Three way set intersection.

Hi this is my first post and thanks in advance.

Im trying to do a three way intersection here is my question.

- Suppose that 25 people attended a conference with three sessions, where 15 people attended the first session, 18 the second session, and 12 the third session. At least how many people attended all three sessions?

So far I have:

First u Second = First + Second - |First n Second|

First n Second >= 15 + 12 - 25

First n Second >= 8

25>= (First n Second) u Third = (First n Second) + Third - |First n Second n Third|

and when I do all of this i get First n Second n Third >= -5 !!

Ive been reading everything to figure this out.