November 1955 saw the start of Metropolitan Series III (NK3) production (Commencing with VIN E21008 on 28 November 1955). A redesign at this time saw the Metropolitan's B-Series engine increased in capacity to 1,498 cc (91.4 cu in) (as used in the Austin A50 Cambridge). Polished stainless steel sweep-spears on the body sides allowed a new two-tone finish to be incorporated, which had the cosmetic effect of lowering, slimming and lengthening the car. The new exterior colors were P905 "Caribbean Green", P910 "Sunburst Yellow", and P911 "Coral Red" with P909 "Snowberry White" as a contrast. The grille was also redesigned, and the hood had its non-functional hood scoop removed. American Motors changed the designation to "Metropolitan 1500" to differentiate it from the earlier 1,200 cc (73 cu in) models. The interior was also changed to incorporate a "houndstooth" check material for the seats trimmed with white vinyl. The dashboard was also now painted black, rather than the body color as was the case for Series I and II Metropolitans.

Series 3 hood ornament

In September 1957, AMC announced that it was dropping the Nash and Hudson brand names. The Metropolitan was subsequently marketed under the "Metropolitan" name only, and sold through Rambler dealers. It is believed that the Nash and Hudson Grille medallions were discontinued around October 1956 (VIN E28326); they were replaced with the "M" style grille medallion.

Price new in 1957 Hard top: £475 + £238 17s purchase tax Contvertible: £482 10s + £242 12s purchase tax