The unhappy triad (of O'Donoghue) is combination of a meniscal tear, medial collateral tear and anterior cruciate ligament tear of the knee all resulting from the same injury.

O'Donoghue's triad

Cruciate ligament tears

What makes you think you might have torn your cruciate?

Multiligament instability of the knee

An overview of the types of multiligament instability seen in the knee.

Complex injury involving the collateral ligament

When several structures are damaged, decisions about the collateral ligament become more important.