What should I add here to make it work? The output should be 3 * 5 = 15V.
what i should add here to make it work .... the output should be 35 =15v*
It looks like you have got U5's inputs swapped around so it's acting as an inverting comparator and not a linear circuit with positive gain: -
You should also ensure that the op-amps have enough power supply headroom to be able to generate +15 volts on their output. Maybe retest but using smaller voltages that give a product that is less than 10 volts.
I also think that your output stage needs some refinement to make it a unity gain buffer. For instance, I believe that the 100 kΩ resistor (R7) should connect to 0 volts and the node between D3 and R7 should be the output that feeds the unity gain buffer.