The second night of action at the WFCU Centre opened with the United States' scintillating 4x50m medley relay win.
Hungary's Hosszu, a treble gold medalist 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.
Hosszu nabbed her second gold of the championships in the 200m fly, pulling away for a decisive victory over Worrell in 2:02.15.
Worrell's American record of 2:02.89 gave her silver less than 15 minutes after her swim in the US relay triumph and China's Zhang Yufei earned bronze in 2:05.10.
Hosszu, winner of the 400m medley on Tuesday, topped the podium again in the 100m backstroke.
Her time of 55.54 didn't approach the world record she set at the 2014 worlds, but put her seven-tenths of a second in front of Canadian Kylie Masse (56.24), with Britain's Georgia Davies third in 56.45.
Davies edged long course world champion Emily Seebohm of Australia for bronze.
Park produced a championship record of 1:41.03 -- breaking the mark of 1:41.08 set by Ryan Lochte in 2010 -- to win the 200m free.
South African Chad le Clos stormed from last place at the midway mark to take silver in 1:41.65 and 400m free silver medalist Aleksandr Krasnykh of Russia was third in 1:41.95.
Mitch Larkin, shut out of the medals in the 100m backstroke in Rio, retained his short course world crown in the event -- to go with the long course world gold he captured in 2015.
Larkin, equal sixth at the 50m mark, stormed home to win in 49.65sec -- edging Russian Andrei Shabasov at the wall by four-hundredths of a second.
China's Xu Jiayu took bronze in 50.02.
There was a measure of redemption after Rio disappointment for Germany's Marco Koch, who pulled away in the closing meters to seize gold in the men's 100m breaststroke.
Koch, a 2015 long course world champion who was seventh in the 200m breast in Rio, won in 56.77.
Russia's Vladimir Morozov took silver in 57.00 and Italy's Fabio Scozzoli won a fierce battle for bronze in 57.04 - one one-hundredth of a second in front of Brazil's Felipe Franca Silva.
Russia won an entertaining mixed 4x50m freestyle relay in 1:29.73, with the Netherlands second in 1:29.82 and Canada third in 1:29.83.