The H-3 was a type of Pacific express locomotive introduced by the MKT in 1915. Compared to the H-2, the cylinder diameter had been increased from 24 to 25 inches, but the cylinder stroke and driver diameter remained unchanged. The firebox now had more heating surface thanks to four arch tubes and the boiler barrel had more tubes and superheater flues. A first batch of twelve was built by ALCO-Schenectady in 1915 and designated H-3a. While these had a boiler pressure of 200 psi, the ten H-3b delivered in 1917 were operated at 210 psi. Additionally, these were oil-fired.

In 1920, the production of the class was taken over by Lima. This resulted in ten H-3c which were virtually identical to the H-3b. Finally, in 1923, Lima delivered five H-3d. These had larger tenders with 10,000 instead of 8,000 gallons of water, while the oil supply was still 4,033 gallons. Later a part of the class was rebuilt with Boxpok drivers. They were retired in the early fifties.