conditional Plotting in gui

How Can I get the status of a callback. This status should show if a callback is executed or not yet excuted. The real problem as follow: Funtion F1 is plotted on Plot1 and function F2 in Plot2. The Function F3 = F1+F2 is plotted on Plot3. But for initial condition Plot1 has F1 and Plot2 has F2 and Plot3 is empty. F1 can be changed and should update only plot1(plot3 should be empty all the time). Once the "ADD" button is pressed then F3 = F1+F2 is plotted on Plot3 and any modification of F1 or F2 should affect both plot Plot1 and Plot 3

Thanks