It is just down to economics;
A CAT3 autopilot, not even considering the autoland, autobrake capability is VERY expensive and time consuming in certification. As a consequence some manufacturers elected to certify CAT3 approaches manually using a HUD;
This is the case of Dassault and the Embraer 145 (i do not know the others).
Example : Falcon 2000 EASy
CAT2 ------> Autopilot flown HUD stowed minimas 110 ft
CAT2-----> Manual flown HUD 2 deployed minimas 100 ft
CAT3-----> Manual Flown HUD 3 Deployed minimas 50 ft
If the HUD is deployed during CAT 1/ CAT2 approaches and autopilot is ON, the approach guidance cue will be displayed down to 100 ft in CAT1 and autopilot disconnect on CAT2 (50ft the latest); leaving only raw data displayed in the combiner.
The flare cue which is represented by a + + will show within specific conditions, at least 10 seconds before touchdown
and not less than 20 ft...
