Hi and welcome.
If you are using a Sky box, then all BBC channels are already available on the Sky channel list and so you do not have to add them to other channels (unless you are using an ROI Sky card - in which case just boot the sky box without the sky card and all BBC channels wil be available on the list!).
BBC 3 is on Sky channels 115 and BBC Four is on Sky channel 116 - so no need to add them to other channels.
Perhaps this is the reason why they have disappeared from oyur "other channels list".
And a few makes of Sky boxes do tend to lose channels in the "other channels" lsit from time to time, especially if there is a apower blip or cut.
BBC3 and BBC4 are part time channels only - from 7pm UK time only. So if you try and watch them before then you will not get anything.
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