First, the bad news. He shows the circuit running on a single 9 V battery. It will not. It needs both plus and minus supplies. This can be two 9 V batteries in series, with the common point as the circuit ground. Note that there is no ground symbol anywhere in the schematic. This tells us a lot about the design and the designer. The schematic in the comments section is much better.
You do *not* adjust the signal amplitude by varying the power supply voltage. On paper it will work, but in practice it can introduce clipping and other distortions.
Each of the three signal output points is an opamp output, which is a low impedance point. You can add three pots, one at each signal output to GND, to give you independently adjustable outputs. The pots should be 10 K or higher. Note that the pot value also is determined by what the load on the wiper is. IOW, what the variable output is driving.
Now, about the circuit ... This is not a good one. The problem is that there is no feedback from the triangle output to the square wave generator. Without that, as you adjust the square wave frequency the amplitude of the triangle wave will change. At low frequencies, the peaks of the triangle might be clipped. Also, unless the components are *perfectly* matched, the integrator output will acquire a DC offset that eventually will be so large that the opamp output is saturated and there is no signal. Both issues can be fixed by rearranging the two circuits. More on that later.
The third stage is a single-pole lowpass filter. Because it is not adjustable, its filtering action (rounding the triangle into a sine) will vary with frequency. This is much more difficult to fix.
The fourth stage is a simple output buffer. It is ok as is, although I would have gone with a non-inverting gain stage so the sine phase is the same as the other two waveforms and the amplitude is closer to that of the other waveforms.
Here are links to better triangle/square circuits:
http://www.interfacebus.com/triangle-wave-oscillator-circuit-schematic.html
https://saaqibs.blogspot.com/2014/09/triangle-square-wave-oscillator-circuit.html
https://forum.allaboutcircuits.com/...ing-a-simple-triangle-wave-oscillator.156475/
http://www.piclist.com/images/www/hobby_elec/e_ckt16.htm
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