Lucas III originated from the Zala branch of the extended gens (clan) Péc, which had large-scale possessions in several counties of Transdanubia, in addition to other parts of the Kingdom of Hungary.
[3] Sometimes before 1291, shortly after the death of Bishop Peter Kőszegi, Lucas and his brother Apor successfully besieged and captured Tátika Castle (located near Zalaszántó), which belonged to the property of the Diocese of Veszprém.
[4] As a supporter of Andrew III of Hungary in his efforts against the oligarchic powers, including the Kőszegis, Lucas functioned as Master of the treasury in the court of Queen Fenenna of Kuyavia in 1291.
He was present when the envoys of the two realm met near Pressburg (today Bratislava, Slovakia) to conclude the war in August 1291.
He was again made ispán of Zala County by 1298, but it is possible he held the dignity without interruption since 1289, as Gregory Kőszegi could usurp the title in his documents.