Reflecting over the x axis means to perform the following tranformation:
So, you maintain the x coordinate, and change the sign of the y coordinate. The vertices become
Translating 3 units to the left means to perform the following transformation:
So, you maintain the y coordinate, and decrease the x coordinate by 3. The vertices become
So, as you can see, after these two transformation, the vertices A, B and C become A', B' and C'.