My 100 amp box/panel seems to have lost the third leg
I don't imagine you just opened up your panel for no reason; I'm guessing appliances have stopped working in your house? 

You could bust the seal on the meter pan and measure the lugs there.  But there's really nothing to break in there, so I'm guessing you'll see the same exact measurements... so it's not worth the paperwork to break the seal.  

Call your power company and report an outage

The problem is almost certainly beyond the meter, probably at the service drop or up on the pole. Either way, you don't mess with the service drop -- that's the power company's job.  

They should do this repair for free.