Yes, you can build a super computer...

...with that many Pentium 3's that might actually just make a regular computer.
I remember my old computer was a Pentium 3 with 399 mhz

i don't think you can get a mobo with 15 CPU slots, short of making one yourself.

Most ive ever seen on a commercially available server mobo if I recall correctly was 4

Make some so-so machines, and throw Linux on them.

You can do better



I tried linking my laptop and pc to share cpu power before...
It never worked out.

build a ghetto ps4 and sell to sony for $$$$$$$$$$$$$$$$$$$$