For these past three years, I've been dating only her.
Literal
I [topic-は] this 3-year-period her only [with-と] dating-am.
だけ marks an exclusive limit ('only her, no one else'). The pattern [time period]+~ている expresses an action ongoing for a duration. 付き合う ('to associate with, to date') takes と for the partner. The この３年間 frames the past three years as 'this stretch up to now.'