You can get this very early on, an easy way to get them distracted even if there are no items around is to step out of cover just a little bit until their indicator goes white then go straight back in cover, they'll start walking in your direction but not be on full a alert. Cover can be a wall or a bush but probably safer in a bush.
Then stealth kill from said bush and you'll be all set. That's 1/3. Repeat 2 more times and you're done. This can be done by the 3rd of 4th encounter with enemies in the story if not before. Good luck!