jmc

If exactly two of the triplets are in the lineup, we have 3 choices for which triplets to put in the starting lineup (we can see this by noting that we have 3 choices for which triplet to leave out), and then 11 people to choose from for the remaining 4 spots. So the answer is $3\times \left(\genfrac{}{}{0ex}{}{11}{4}\right)=3\times 330=\boxed{990}$.