Sucrose is a non-reducing Sugar while Maltose and Lactose are reducing due to the presence of free Aldehydic Functional Group in them. In SUgar the Aldehydic Group of both Glucose and Fructose gets condensed and get involved in a Glycosidic Bond. Hence the Reducing Nature of Sucrose becomes deactivatedand non-function due to absence of any functional group.
Sucrose is non sugar of the aldehydic group of sucroseis bonded so it is non sugar.
SUCROSE = GLUCOSE+FRUCTOSE
where as MALTOSE and LACTOSE havind free aldehydic gruops so these are sugar.