Cameron Boyce did not participate due to starring in his own Disney XD show, Gamer's Guide to Pretty Much Everything (2015). On Bunk'd, the reason for Luke's absence is because he's at summer school, though he does eventually make a guest appearance in two episodes.
The Emma-Hazel-Xander love triangle is supposed to represent the Gladys-Morgan-Christina love triangle that happened when Emma's parents and Hazel's aunt went to Camp Kikiwaka when they were 16.
It's confirmed in October 2017 that Kevin Quinn (Xander), Nathan Arenas (Jorge), and Nina Lu (Tiffany) have departed the show and wouldn't return for Season 3 in 2018, and that Raphael Alejandro (Matteo), Will Buie Jr. (Finn), and Mallory James Mahoney (Destiny) replace them, though Quinn's voice remains singing the theme song in the intro. It never is explained in Season 3 as to why Xander, Jorge, Tiffany, and Griff did not return to Camp Kikiwaka.
The events of episodes from Jessie (2011) were never mentioned in any episodes of Bunk'd (2015), nor has Jessie ever been mentioned or seen.