Are those vans/commercial vehicles allowed to use the 1st lane of expressway?

Light commercial vehicles (includes Light Goods Vehicles and small buses not exceeding 3.5 tonnes and seating capacity of up to 15 passengers).
Road 50km/h
Expressways 60-70km/h
Tunnels 50-70km/h.
(Source: LTA)

Usually 1st lane is used for overtaking and emergency vehicles. But it's recommended for the vans/commercial vehicles to use 3rd lane or in other cases, 2nd lane as long as they don't hog the traffic.