I think someone more knowledgeable than me should chime in, but I'll contribute my small amount of experience as a ham radio operator.
You can easily receive
all three bands on one radio. In fact, nearly all VHF ham radio transceivers have the capability to listen on all these bands, as do most scanners.
I think in your question though, you meant transmit
. That one is a little more tricky. There is no technical reason one box couldn't contain the tuner and related circuits to transmit FM on 144-148/440-450 MHz for ham, AM around 27MHz for CB, and FM around 460MHz for GMRS. On the legal side of things, I don't know of any FCC regulation preventing this either, but I could be wrong.
With any such unit though, the buyer would have to possess both
a amateur radio license and a GMRS license to use it. (CB is the only service you mentioned that doesn't require licensing to transmit on) It seems to me the issue is more about sales. How big of a market is there really for individuals who have both licenses mentioned and still want to transmit on CB also? My guess is not very big. Most people are really into one service or another, not all at once. So because it isn't profitable to sell, it isn't made! And if you just want to listen
, not transmit, most scanners on the market, as well as most VHF ham radios, would achieve that.
If you want to build it and sell it though, be my guest! I'll even be your first customer!
Hopefully someone with some more knowledge than me will chime in next though and clarify a little....I'm sure my ramblings are hard to read