me again :-)
If "Hud-3d" is checked in the advanced options, the resulting command line is not "--enable-hud-3d". But the default in flightgear is "--disable-hud-3d" (maybe it was different in previous versions).
That's why "--enable-hud-3d" must be added to the command line. To be compatible with older flightgear versions, maybe "--disable-hud-3d" should also added to the command line.
true has always been the default and still it is :
I:\Devel\FlightGear\src\Cockpit\hud.cxx(321): static const SGPropertyNode *enable3d_node = fgGetNode("/sim/hud/enable3d");
I:\Devel\FlightGear\src\Cockpit\hud_ladr.cxx(51): if (fgGetBool("/sim/hud/enable3d", true) && HUD_style == 1)
I:\Devel\FlightGear\src\Instrumentation\HUD\HUD.cxx(53): _3DenabledN(fgGetNode("/sim/hud/enable3d[1]", true)),
I:\Devel\FlightGear\src\Instrumentation\HUD\HUD.cxx(371): } else if (!strcmp(name, "enable3d")) {
I:\Devel\FlightGear\src\Main\options.cxx(189): fgSetBool("/sim/hud/enable3d", true);