Translation:This athlete is participating in three events.
Actually, "This athlete is participating in three events." translates slightly different. "is participating" is translated into ３種目に「出ています」as it is present continuous. 〜（し）ています。expresses present continuous. 「出ます」is either presence or confirmed future, like plans to participate. Therefore"このせんしゅは三しゅもくに出ます。" should be "This athlete will/is going to participate in three events." or "This athlete participates in three events. "