Are the MAC addresses of Wi-Fi, Ethernet and Bluetooth devices assigned in the same address space? Or are MAC address unique irrespective of the physical layer? Is there a possibility of MAC address collision if Wi-Fi (or Ethernet) and Bluetooth devices reused the MAC address? While routers bridge WLAN and Ethernet LAN, I have not seen any device that bridges Bluetooth and LAN (Ethernet or WLAN). Do such devices exist?
Yes, they're all IEEE EUI-48's (EUI == Extended Unique Identifier), and the IEEE Registration Authority manages the address space to guarantee uniqueness. The first 24 bits is an Organizationally Unique Identifier (OUI). Each OUI is assigned to a particular company by the IEEE. That company can then make 2^24 unique MAC addresses with that OUI as the prefix. It's the company's responsibility to make sure they never program the same MAC address onto more than one device or interface.