I am the megamosk

Calculate average of up to three scores (presence of a score is determined by the gradeN>0 condition).

CASE WHEN Topic1>0 THEN Grade1 +
CASE WHEN Topic2>0 THEN Grade2 +
CASE WHEN Topic3>0 THEN Grade3
ELSE 0 END
WHEN Topic3>0 THEN Grade3
ELSE 0 END
ELSE
CASE WHEN Topic2>0 THEN Grade2 +
CASE WHEN Topic3>0 THEN Grade3
ELSE 0 END
WHEN Topic3 > 0 THEN Grade3
ELSE 0 END
END AS TotalGrades
, CASE WHEN Topic1>0 THEN 1+
CASE WHEN Topic2>0 THEN 1+
CASE WHEN Topic3>0 THEN 1
ELSE 0 END
WHEN Topic3>0 THEN 1
ELSE 0 END
ELSE
CASE WHEN Topic2>0 THEN 1+
CASE WHEN Topic3>0 THEN 1
ELSE 0 END
WHEN Topic3>0 THEN 1
ELSE 0 END
END AS NumberOfTopics

Unfortunately, I do not know how to calculate the actual average (TotalGrades/NumberOfTopics), so I guess I am not the megamosk. Oh well...