While Activision are "proud to be participating in this premier video game event" that is E3, they - like EA - won't have a booth at the show, choosing instead to focus on a community event that'll be held in spring.
Activision aren't skipping E3 entirely; they'll be showcasing this year's Infinity Ward-developed Call of Duty game in partnership with PlayStation:
In June, we’re going to be at E3 showcasing gameplay from Infinity Ward’s ambitious new game. We’re looking forward to sharing exciting new details about the next great Call of Duty game in partnership with our friends at PlayStation. We’re proud to be participating in this premier video game event, but won’t have an Activision booth on the show floor.
We’re also going to be celebrating the community this fall at the Call of Duty Championship, marking the epic conclusion of the first season of the Call of Duty World League, and holding a major community event that will give fans special insight and access to the world of Call of Duty.
There's a good chance that Destiny will have at least some sort of presence at the show, with a "large new expansion" on the way and a sequel on the cards for 2017. Activision are also publishing PlatinumGames' Teenage Mutant Ninja Turtles: Mutants in Manhattan, which is due out this winter, but Call of Duty is the only game they've specifically mentioned for E3.
In January this year, EA similarly announced that they wouldn't have an E3 booth. Instead, they're hosting their own event in the days prior to E3, which will be open to the public.