The source is Anth. Gr. 13.19; I follow Farrington (2012), no. 1.15 and p. 103 note 212-3 for the dating, who in turn based it off of Simonides' (the poet's) career. The poem mentions victories both in the stadion and the pentathlon, and only occasionally ties them to specific victories, so I do not share Farrington's firmness in assessing the Isthmian victories as in pentathlon, though that is the closest aforementioned discipline.
Three victories on the Isthmos, uncertain whether this is still about pentathlon. - PK