2–6–4T no. 456 built by Tampella in 1906 and classified I3 had the distinction of being the first Finnish engine to be superheated. Following favourable trials, a further six almost identical engines were ordered from Tampella, numbered 487–492, and delivered in 1909.
The I3s only differed from the I2s (including no. 456 originally) in having larger cylinders. This difference was the criterion whereby which nos. 487–492 were placed into their own distinct Vk3 class in 1942. No. 456 was a Vk2 class engine until larger cylinders were fitted in 1947 when it too was reclassified Vk3. The I3s were built with carbide gas-lighting equipment, replaced with Aga in 1920s. Nos. 488 and 492 used a Kylälä blast pipe between 1919 and 1920. All the I3s were altered between wood and coal burning a number of times.
Similar to the I1 and I2 classes, the I3s were employed initially on Helsinki suburban services until the introduction of the heavy N1 suburban tank engines between 1924 and 1926. The I3s were then put on short-distance passenger trains on branch lines and shunting duties in different parts of the country. They were allocated to Turku, Karjaa, Helsinki, Riihimäki, Kouvola, Viipuri and Sortavala in the 1930s, being notably employed on the Inkeroinen–Hamina line from the mid-1920s until the late 1940s. Depots to which the Vk3s were allocated after the 1941–44 Continuation War were Karjaa, Kouvola, Seinäjoki, Pieksämäki, Oulu and Joensuu. Duties in the 1950s and 1960s were almost confined to shunting until they were replaced by class Vv13 diesel shunters.