The meaning of congruency is that two shapes which are identical.For example leaves of same tree, Same kind of bulb manufactured by a company, Woman hair if not cut, etc.
Now when you talk about triangles , the congruency has the same meaning i.e the two triangles are identical in shape, size means corresponding sides are equal in length, and corresponding angles are equal.
So, the two triangles can be proven congruent by
1. S A S
2. A SA
3. A A S