what/why 3 processors on one motherboard

I was given a few older computers and computer parts that came out of an old business. i noticed one of them was a lot bigger than the rest. when i took it home and pulled the side panel of to **** the dust off of it. I saw two processors with heat sinks and fans… then saw a slot for an additional possessor setup. The two were pentiem III. I requires 2 power sources. I have searched all over just trying to figure out the specs and or reason for 3 of them….. please help!