SEOUL—Samsung Electronics unveiled its latest high-end foldable smartphones on Wednesday with lower prices than last year’s offerings in a push to expand foldable demand beyond niche devices. The tech giant priced its 5G-enabled Galaxy Z Fold3 with 7.6-inch main screen to start at $1,799.99 in the United States, and clamshell Galaxy Z Flip3 at $999.99—down…