Lee Stevens had a single and double, scored twice and drove in a run for the Expos, who posted their first spring win under new manager Frank Robinson.
Orlando Cabrera also had two hits for Montreal.
Yoshii, bidding to earn a spot in the rotation, followed starter Zach Day with two impressive innings. Day allowed a run and two hits in two innings.
Baltimore starter Sean Douglass allowed two runs and two hits in two innings. He walked two before giving way to John Stephens, who allowed the three runs in the sixth.