Hungary's Hosszu, a treble gold medallist at the Rio Olympics, captured her second and third gold medals of the championships in Windsor, Canada, with convincing wins in the 200m butterfly and 100m backstroke.
South Korea's Park, who fought to swim in Rio after serving a drugs ban but came away from the Olympics empty-handed, added 200m freestyle gold to the 400m free title he won on Tuesday.
The second night of action at the WFCU Centre opened with the United States' scintillating 4x50m medley relay win in a record-shattering 1min 43.27sec.
Alexandra De Loof, Rio Olympic gold medalist Lilly King, Kelsi Worrell and Katrina Konopka set the first world record of the championships, erasing the previous record of 1:44.04 set by Denmark at the 2014 short course worlds in Doha.
They nabbed America's second gold of the meet, after Worrell led the women's 4x100m free relay to gold on Tuesday.
Italy were a distant second in 1:45.38 and Denmark third in 1:45.98.
King returned to win individual gold in the 50m breaststroke in 28.92sec – getting the better of world record-holder Alia Atkinson of Jamaica (29.11) with fellow American Molly Hannis third in 29.58.
