What does it mean when free T3 (liothyronine) is abnormally high but free T4 and TSH are in the normal range?

Consult physician. Sometimes diseased thyroid gland will start producing very high levels of t3 (liothyronine) but still produce normal levels of t4. Therefore measurement of both hormones provides an even more accurate evaluation of thyroid function. Interpretations of TSH level depends upon the level of thyroid hormone; you should consult your physician and maybe repeat tests at future time.
How high? The ranges are actually set so that a few percent of healthy folks fall outside at either end. Your treating physician is aware of this. If free t3 (liothyronine) is extremely high and the others labs suppressed, you may have a tumor making t3 (liothyronine) on its own. If you feel well, have no mass, and the other labs are normal, and your physician is good with it, don't be concerned.