Starting from the left, look at each logic gate's inputs. For the leftmost gates, these inputs are labeled by variables (A, B, or C). Over the line that leaves the gate on the right-hand side, write the inputs joined by the gate's operation.
For example, in the first picture, the top-left gate is AND. Its inputs are A, B, and C. So, over the line that leaves the gate on the right, write A AND B AND C, or A + B + C, depending on your notation. Repeat this moving from left to right.