It was decided to acquire specific passenger and specific freight locomotives for the Riihimäki–St Petersburg Railway. No fewer than 19 locomotive works made offers and from these the board of the railway ordered a batch of ten locomotives from the Scottish firm of Dübs for its passenger services. These were completed the following year as nos. 11–20; they were classified A3 in 1885. Four more, numbered 44–47, were constructed in 1871. Apparently, these locomotives were considered successful as an additional eleven, nos. 178–182 and 222–227, were built in 1893 and 1898 respectively.
The A3 boiler had a copper firebox and brass tubes. A number of features differentiated the A3s from the A1 and A2 classes, such as the Stephenson inside valve gear and lower, straight running plates under which the sandboxes were fitted. Originally, nos. 11–20 and 44–47 had no engine braking apart from a screw brake applied to the tender wheels but nos. 178–182 and 222–227 were fitted new with steam brakes applied to the driving wheels. Oil lamps were used. The frames of nos. 44–47 were strengthened, making them slightly heavier than earlier members of the class.
The later batches of A3 locomotives built in the 1890s were identical to the earlier class members but were of sturdier construction. New technical innovations brought train heating equipment and the Westinghouse air brake. Nos. 222–227 were also equipped with Nathan displacement lubricators, and their boiler working pressure was raised from 8.4kg/cm2 to 10.6kg/cm2. Oil lamps were used on nos. 178–182 but nos. 222–227 received carbide gas lighting.
Certain members of the class were later fitted with a replacement boiler with a steel firebox and tubes. Air brakes and train heating equipment were even fitted to the oldest locomotives in the series. Practically the whole class was coal fired between 1908 and 1911, and nos. 222–227 burned coal for short periods in their latter years.
The A3 class engines were mainly employed on passenger duties in southern Finland. When first introduced, they handled all forms of passenger services but were relegated to light or medium passenger trains in the early 20th century. However, from the First World War onwards, when trainloads were constantly on the increase, their lack of power limited them to light passenger or suburban services and even occasional lighter goods workings. Some of the A3s were finally relegated to permanent way duties.
When the nos. 11–20 series was first introduced, four went to St Petersburg, three to Viipuri, and the remainder for various periods to Lahti, Riihimäki and Kaipiainen sheds on the Riihimäki–St Petersburg line. Once the H1 and H2 classes were introduced in 1898, the St Petersburg-based A3 engines were gradually transferred to Turku, where six of this batch remained until withdrawal. Two of the other four, nos. 11 and 12, spent their entire working lives at Viipuri.
Nos. 44–46 were allocated to Helsinki, St Petersburg, Riihimäki and Viipuri until the turn of the century with brief periods at Turku before finally staying there until scrapping; no. 47 was always a Viipuri locomotive. Nos. 178–182 were allocated to St Petersburg MPD between 1893 and 1900 but four of the five were moved to Riihimäki once the H1 and H2 class engines entered service; the fifth of the batch was transferred from St Petersburg to Turku in 1904. Decreased activity following the First World War resulted in short periods at Helsinki, Karjaa, Turku, Toijala, Mikkeli, Kuopio and Viipuri.
Nos. 222–227 were allocated new to St Petersburg but after the H3, H4 and H5 class locomotives replaced them in 1905, they could be found at various depots in southern Finland until 1919 after which they were gradually transferred to Turku. They became superfluous on account of their lack of power, with the oldest A3 engines being condemned between 1911 and 1913, and the newer ones between 1927 and 1930 without receiving new replacement boilers.