As discussed, operators tend to reduce the 5% to 3% when they have restrictions on the amount of fuel they can take either due to MTOW or MLW limitations.
Briefly from AMC:
3. Contingency fuel, except as provided for in (b), which should be the higher of:
i. Either:
ii.
(A) 5% of the planned trip fuel or, in the event of in-flight replanning, 5% of the trip fuel for the remainder of the flight;
(B) not less than 3% of the planned trip fuel or, in the event of in-flight replanning, 3% of the trip fuel for the remainder of the flight, provided that an en- route alternate (ERA) aerodrome is available;
or an amount to fly for 5 minutes at holding speed at 1 500 ft (450 m), above the destination aerodrome in standard conditions.
Location of the Fuel En-Route Alternate (Fuel ERA) Aerodrome (AMC2 CAT.OP.MPA.150(b))
The fuel ERA aerodrome should be located within a circle having a radius equal to 20% of the total flight plan distance, the center of which lies on the planned route at a distance from the destination aerodrome of 25% of the total flight plan distance, or at least 20% of the total flight plan distance plus 50 NM, whichever is greater. All distances should be calculated in still air conditions.
The applicable weather minima are the same as for destination alternate aerodromes: