Can I use a PC/104plus board on a PCI-104 connector
You say that all three standards use the PCI bus, but the wikipedia article states that PC/104 bus uses the ISA bus, PC/104-Plus supports both ISA and PCI and PCI/104 supports only PCI bus.

From the PC104 website, the difference between PC/104-Plus and PCI/104 is that the AT &amp; XT (ISA bus) connectors have been removed for increased board real-estate.

So the answer appears to be yes, they are compatible since the 4 X 30 PCI interface connectors are identical.

Connect Tech, amongst others, make standard PCI adapters which support both PC/104-Plus and PCI/104 cards with the same interface socket.