noun - A three-fer swim (or threefer) is when 3 people swim together shoulder-to-shoulder or together in a formation in an open body of water, usually comfortably going at the same pace for the entire swim or extended periods.
 Alternate meanings
A three-fer swim can also refer to 3 swimmers swimming at the same time and at the same pace during an open water swim using the same kayaker or paddler. As in, "Lexie and Parks did a twofer swim around Naples Island on Saturday."
A three-fer swim can also refer to 3 swimmers on a marathon swim or channel swim going the same pace while sharing the same escort boat and crew. As in, "Marcia and John did a twofer in the Catalina Channel."
A three-fer swim can also to one swimmer competing in 3 different races on the same day at the same venue. As in, "He first did the 1-mile race in the morning, the 2-mile race later, and then finished his three-fer when he did the 3-mile Gatorman race."