During the mobilization prior to the Winter War, the 6th Division was placed in the reserve of the Commander-in-Chief Carl Gustaf Emil Mannerheim. The division consisted of reservists mainly from Satakunta. On 19 December it was attached to the II Corps. The 6th and 1st Divisions were to attack the advancing Soviet forces near Summa. The attack began on 23 December but was a failure.
In the beginning of January 1940 the division's name was changed to the 3rd Division. The reason for this was to confuse the enemy, but it is uncertain if this had any effect.
In February the Soviet forces began bombarding the front lines at Summa with artillery fire. On the morning of February 11th the attack began along the whole front of the exhausted 3rd Division. The Soviets had concentrated nine divisions and five armoured brigades of the 7th Army at Summa. The Finnish lines broke on the January 13th and the forces retreated to the Intermediate Line (Väliasema).