They only have ten forwards on their roster; I think St Martin missed some time with an injury, but I saw her on the ice last night. So, yeah, only three forward lines is what they have.
Adding: the UW team twitter page pretty much always posts line charts for both teams, and looking (closely) at the Ohio St chart, they list 'games played' for each player. 19 games played total for them going into last night, St Martin shows as having played in only 15 games played, and Makenna Webster - having missed games while playing field hockey - has played in only 9 games this year. So it's possible there was a game or two when they had only eight true forwards dressed. I don't know if one of the D players might have switched to forward to give them a full three lines.