Thyroid function in "normal" range. Took 2.5mcg T3 (liothyronine) for 2 months, then stopped. What potential harm did I do? What should I do now, afterward?

T3 (liothyronine) use for two month. The question I have to ask you why were you prescribed T3 (liothyronine) and why did you take for two months with normal thyroid profile. Having said that you did not cause and short or long term problem and once you stopped it, it is out of your system for good. So with normal thyroid function which incldes t3, (liothyronine) t4, and tsh. If all are normal there is no indication for any hormone therapy.
Don't do it again. Although the dose was very low and you most likely didn't caused permanent damage, I caution you from doing it again. The risk of hyperthyroidism outweighs any benefits you expected from taking extra hormone. The hormonal system is very sensitive, when you add extra hormones to the body when it doesn't need it, the body will shut itself down and might not be able to wake up once you stop the pill.