To unlock the "THREE AT JERICHO" trophy in Detroit: Become Human, you must make sure all three principal characters survive until the "CROSSROADS" chapter and ensure Connor learns the location of Jericho during "LAST CHANCE, CONNOR..."
Both Kara and Marcus can die in several chapters, though in most cases it's easy to make the correct choices and pass the QTEs if you are playing in Casual mode. The trickiest to survive is probably "ZLATKO", but you can follow the guides here to make sure Kara makes it out of that one:
The trophy will unlock when you reach the end of this long chapter and arrive at the flowcharts.