***Answer***

Ah, let me see, I agree real time FFT is too complicated and over killing. 

Goggle tensorFlow trained on very limited, say only 3 very short, 2 seconds long target tunes might be easier.  But it might be a bit tedious to train the stupid tensorFlow ears.

Easiest might be to use Rpi and ADC to do "structured" and "statistical" pattern recognition, as summarized blow: 

> (1) Chop/structured the converted digital signal string into 4 or more sections, 
> 
> (2) Get the statistically overall, moving time max, min, and avg values of each section,
> 
> (3) Compare/contrast/correlate the above overall/section values with that of
> the three target tunes to pick your educated/calculated guess.

You might need to try and error to trade off recognition time and successful recognition rate etc for optimal parameters of say, chopping number, moving time period etc.

For real time streaming audio hobbyists, I would recommend dirt cheap PCM1802 (Ref 1).

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*Adding bells and whistles*

If you find the project too simple or too easy, you can add some bells and whistles like using the also dirt cheap PCM1808 DAC studied in the project to play back what your son have played.

--

*Letting the servos dancing*

It won't be too more difficult to let a 16 servo [robot girl to dance][1] along, while playing the guessed tube.  For this part, you might use the other dirt cheap chip, the 16 channel PCM/servo controller PCA9685.  

Warning: No guarantee that your son's school mates might not become over jealous, or he not over worrying which pretty girl to invite to the graduation party.

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***References***

(1) [How to use Rpi python to control PCM1802 24-bit HiFi stereo ADC and MAX4466 microphone amplifier][2]

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***Appendices***

(A) *PCM1802 Features and Block Diagram*

[![pcm1802][3]][3]



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*End of answer.*

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  [1]: https://www.youtube.com/watch?v=As99fxCfldo
  [2]: https://raspberrypi.stackexchange.com/questions/107778/how-to-use-rpi-python-to-control-pcm1802-24-bit-hifi-stereo-adc-and-max4466-micr%2044Aier
  [3]: https://i.sstatic.net/5Kgal.jpg