Perhaps you didn't mean to say "signaling"? 3-volt signaling and 5-volt
signaling -- as in sections 4.2.1 and 4.2.2 of the PCI spec -- are mutually
exclusive environments that can never mix. (Like mixing TTL and ECL.)
What you can do, however, is use a device that is powered by 3.3V, and
outputs 0/3-volt output levels, on a PCI bus designated as a 5V
signaling environment. This is what you referred to as "3-volt level
signaling and '5-volt compliant.'"
This arrangement is permitted because the output voltage specs of the
5V signaling environment are wide enough to encompass what you might
call 3V switching levels, even though they are indeed "5V signaling
environment" PCI levels.
And by the way, the output levels of these 3.3V-powered, 5V signaling
devices is rather similar to good old TTL, which, despite having 5V
power, might drive only as high as 2.4V in the high state. The 5V PCI
switching levels are essentially TTL levels.
There are other differences besides the lack of clamping diodes that
differentiate 3.3V signaling devices from 5V signaling devices, but that
is one of the biggies.