Why does BBC THREE and FOUR keep disappearing from my Computer card, but not fro
BBC THREE shares it's broadcasting capacity with CBBC and BBC FOUR with cBeebies. When the television channels are off-air, they are replaced with a small text service that displays the a message that you see on your set-top box.
However, your PC card operates differently, as it has no "MHEG" facility, it simply ignores all the channels that are just text based (you just get a list of TV and Radio channels).
This means that if you scan for channels in the daytime you will be missing BBC THREE/FOUR.