You can now choose your preferred method using the new ‘Photo Capture Mode’ setting, see changelog in this thread [OPEN BETA] Litchi Hub - #340 by vico
Alternatively you can also set the Capture Mode to ‘Manual’ then remember to start the capture manually.
The Phantom 3 series’s onboard waypoint engine (aka DJI engine) does not natively support interval capture. While Litchi technically could start an interval capture at the start of the flight (simulating you pressing the take photo button while in Interval photo mode), currently that’s also not supported. It was never implemented as newer drone models support it either natively or via the Litchi waypoint engine (also not supported for P3 series).