I have not heard of three way calling but what I think you mean is this.
Get 2 polystyrene cups and make a hole in the bottom of each just big enough to thread a string through then tie a couple of knots in it to keep it there, about ten feet long string. two people can then talk to each other by one holding the open end of the cup to thier ear and the other one talking into the other cup. The string must be kept tight and not touch anything else the sound will travel through the string.
Mobile phones work a lot better!