if 2 people split a drink, there is one drink and 2 glasses, in order to be fair, one pours and the other chooses
how to do it for a 3-some?
This is actually a very famous problem in fair division theory!
Say that the three are A, B, & C.
Allow A to pour a portion of the bottle into a glass.
Now B has the option to decrease the amount by pouring some from that glass into another glass.
BUT B has no obligation to do so, B may pass.
Now C is given the same two options.
The last person to touch the first glass gets the glass.
Now we have a partial bottle or a partial glass.
Now you are back to a two person fair division.
Let them divide the remains.