Thanks to Terry Leese we now know that armed with the right equipment the Vienna Philharmonic horn players are able to play the low notes in Mahler 3.
If they play a Jungwirth horn they can extend the 3rd valve slide.
They can use a special additional 3rd valve on their Yamahas.
They can ask another horn player to use a Bb or A crook to help them with the pedal F and pedal E.
They can use a four valve 'beasty bass' Engel horn.
These are all very interesting and would obviously work. Are we to deduce from this that without any of these helpful gadgets a horn player in the Vienna Philharmonic would be unable to play the notes written by Mahler and would therefore leave them out or just play the 'wrong' notes? So confusing! There must have been so many concerts and recordings with these notes omitted or played just plain wrong if that is the case.