First of all, 3 cameras is a lot to carry. I don't carry around 3 cameras at all times unless I have my bag on me. Typically, I always have 2 cameras on me: my vlogging camera and iphone camera.
Does the iphone camera really count? For me yes, because that's the thing I use it for most. I've finally trained myself to not mindlessly scroll when I'm bored, and hopefully I can keep that going.
Any time I go to work or on a trip I'll have my main 3 cameras and maybe a GoPro (so I guess 4):
Canon EOS-M (for vlogging)
iPhone 6S Plus w/ lens
They all have their jobs, which is why I like have them all at the same time. The D800 is too big to constantly vlog with, however the photo quality is leaps and bounds better than the iPhone. The Canon EOS-M is for quick (despite it being a pretty slow camera) videos and some snapshots here and there.
I used to vlog with the iPhone entirely and I still will every once in a while, but vlogging with a camera that's constantly getting notifications and calls is a pain. Also the mic is pretty weak.
I honestly prefer taking photos with my iPhone, especially for Instagram. The D800 just gives me so much more freedom with printing and pushing photos to their limit. I would love to one day replace this hulking mass with a smaller full-frame mirrorless, but for now the D800 is more than I need.
I don't like having 3 cameras on me at all times. I would prefer to only have one that does it all. That said, I feel ready for almost any situation with my current setup, because I hate feeling unprepared.
It also helps that I have the best camera bag in the world that fits them all and maybe even an additional film camera.
I'm trying to get all my stuff together for NYC, because I know I'm going to want to have my main gear with me at all times. I'm kind of dreading carrying all that....