We have had these channels magically re appear on re-tuning earlier this week.
Loss of channels like this is almost always an antenna/cabling fault. It's possible you got water in the coax (heavy rain recently) which reduced the signal on TV3's mux and now it's dried out restoring normal signal strength.
Loss of channels is a common indication that your aerial and set up will need to be maintained and aligned correctly, as overtime a small shift of the aerial due to wind / and or snow can affect a group of channels. Have a check to see if the aerial is tightened correctly and is directly hitting the line of sight of the transmitter.