First on the list on that
bustimes page (as it should be, because it's the lowest route number!) is the M3 Poole to London service - but it appears that it's no longer running, as it doesn't show up for booking on the
Megabus site. (I think it was run by Heathrow-based operator, which took over after the previous operator Bournemouth-based Yellow Buses collapsed in the summer of 2022.)
Having just had a bit of a search around I've found this interesting
Heathrow National Coach Map PDF, which is dated 6 November 2023 and marked as
Version 92. I wasn't previously aware of its existence. Out of interest I looked on the Internet Archive's
Wayback Machine for
previous versions - the last one saved on the archive before Version 92 is
Version 75 dated 29 January 2023, so it looks like the map does get updated regularly.
Anyway, the point being is that the Megabus M3 route is not shown on V92 of the map from November last year - so looks like the M3 has not been running for at least a couple of months now.
[Edit - removal of stray apostrophe!]