Cat III
The full capability of Cat III is Cat III E4.
Anything less is a downgraded situation.
Full capability means that, if aircraft is certified and fully serviceable and crew is certified and authorized, Cat III b without Decision Height is possible with RVR of 75 m.
A downgrade to Cat III E3 or Cat III D4 could impose limits such as minimum RVR 125, or 200 m, or Decision Height 15 ft, or 25 ft, or Cat 3 a, DH 50 ft and RVR 200 m. You would have to check your operational manuals for details.