3.5 mm connections. Three computers -> two speakers...

Are these amplified speakers?

Bob

 

Then you can combine all the outputs from the computers with a resistor (1K might be a good choice) to each speaker input.

i.e.

out-1 ----\/\/\/\/\----+
out-2 ----\/\/\/\/\----+----- in-1
out-3 ----\/\/\/\/\----+----- in-2

Bob

 

Additionally, if one of the computers will always be on, you can take the output of the other computers and run it into the Audio Input of the 'main' computer.
From here, you simply set a couple options to 'listen' to the input... and any audio being fed into the main computer from a gameboy, kids piano, or another computer will play through the main computer to the speakers

This also gives you the ability to save or record audio on the 'main' PC.

Many PCs are now shipping with 6, 3mm jacks on the back side:
Green - Default Speaker Out
Pink - Default Microphone
Blue - Default Aux-In
Black - Orange - Grey - Default 5.1/6.1 Speaker Outputs.
These computers typically allow the jacks to be 're-assigned' so you can use the Black, Orange, and Grey 'outputs' as inputs. You can tell what jacks can be reassigned much easier if your computer prompts you for what type of device you plug in. (This is typically done with an application bundled with the RealTek audio device. Look on your task bar for windows... I am unsure how to proceed if you are using MAC or Linux other than googling)

 

KVM or some other remote access app.
That said, you can independently control the volume of all the 'add-ons' but only up to the current max volume of the master computer.
At least while on the master computer you can control the volume of anything.

The DIY method of joining the signals with resistors works well, but you can only change volume on the original source.
Only other method is a mixer... You can try to build one, or buy one. Of course even the mixer method requires that you change the volume from one specific location which defeats the purpose. You could just as easily use the other computer as a mixer.