If you're male, your pocket (assuming your smartphone fits); preferably not loose pockets though. If you're female, then in your handbag, but watch out in crowded areas. Those three valuables are small enough and are things you're likely to need during the day/night anyway. When travelling with your backpack, I'd keep those things in the bag I'm always going to have with me (e.g. cabin luggage/under-seat luggage, not checked or luggage you leave out of sight).
Many hostels offer lockers (so don't forget to bring a padlock, and a decent sized one, not a tiny bag-style padlock as they won't be thick enough to provide any protection) so if you don't want to carry your camera around (e.g. using your phone instead to take photos), stash it in there. If they don't, they often let you store it at the front desk if you ask nicely (and if you trust them).
Make sure you have copies of your important documents so it won't be the end of the world if you lose them. Most travellers have these devices anyway, so you won't stand out, but consider bringing an old phone instead just in case you do part ways with it somehow.
Finally, remember to periodically backup your photos online as you go along! This might mean you'll spend a morning or an afternoon in the hostel instead of being out and about, but it'll be worth it. You don't want to have an amazing trip only to lose your camera on the last few days.