First off, I would put away the D60 and the D5100, and use the D7100 exclusively. Then, go here and update the camera to the very latest firmware, which has some important firmware fixes.
Nikon | Download center | D7100 Firmware After you have that updated, then go and get the Distortion correction firmware updates from this page
Nikon | Download center | D7100
One camera will be a big help. Just the one camera. The one with the best controls and the biggest, best, clearest viewfinder. The D7100.
What to learn? I dunno...I think the exposure triangle as an issue, as a concept, as
a thing to worry and fuss over, is a bit overrated with modern Nikons, which in raw mode have terrific post-processing image recovery potential; expose to the right USED TO be a huge, huge deal, 10 years ago...with the newest sensors, not nearly so much. it's trivial to "lift" the shadows in software with a newer Nikon camera like a D7100. So, don't fixate on the exposure triangle. Fixate on your pictures!
You have a camera that can shoot AUTO ISO in MANUAL exposure mode; this is something Nikon does better than most any other camera. Set an appropriate Manual mode exposure as far as shutter speed, and f/stop, for the desired creative end, and then set the AUTO ISO custom setting up properly. Cap the maximum ISO at say, 3,200. Set the minimum shutter speed as appropriate: 2x the focal length for short lenses, 3x the focal length for anything over 135mm. Start shooting.
Or as an alternative, shoot for a month in Programmed Auto mode, which is by the way, shiftable with your right thumb and the rear control wheel!!! See what the camera thinks is the right exposure using Matrix metering and Programmed mode. Work on composing good photos, rather than on the mechanics and the smallest details. Make it like driving your car: focus on the traffic, don't fixate on the speedometer or the gas gauge or the radio...just
drive the danged car. Stay out of accidents. Don't get caught speeding. Blend in well with traffic. Pay attention, be responsible.
SIlhouettes, backlighted subjects, side-lighted subjects, early and late-day angled lighting,shoot it ALL! For snowfields or foggy day lighting set the meter to + 1.5 EV, for black cats set the meter to Minus 1.7 EV, use the programmed Auto mode, shoot a photo or two or three and review it looking at the histogram and then dial in + or - Exposure Compensation as needed, since the D7100 makes this easy and logical.
You have a d-slr: you can shoot an image and in 3 seconds review it and determine what the heck is right, or wrong, with what you shot. Use the power of the force, Luke.
SHOOT SOME TALLS!!!! On portraits, learn that if the subject is taller than it is wide, the natural framing is a TALL. Same with trees and telephone poles and waterfalls, those are talls most of the time. Not always...but a lot of the time.
LOOK through that good viewfinder, and scan the composition with your eye, moving the eye by mental command in a counter-clockwise circle, around the frame. Is everything that you want in the photo in the composition? Is there something you do NOT want in there? If so, re-frame, or move and re-frame.
Spend a bit of time taking photos that depend on split-second timing. Find a place where cars, or people, literally move past a gap or opening, and snap images as the people or cars are precisely in the middle of that gap. Row of parked cars and a sidewalk? Shoot pics of pedestrians as they are right smack dab in the middle of the gaps! ONE shot at a time, not motor driven sequences. TIMING.
Work on timing,and learning the D7100's exact mirror up/shutter timing.
Shoot plenty of images. Don't be restrictive. Walk around, find a subject, and then shoot 20 pictures of it.
Twenty frames. Minimum. Then, you're allowed to move on. Many beginners do not shoot enough, do not explore a subject anywhere near enough, and come back with junk. Lots of junk, in 2's and 3's. Shoot 20. 30,40. Learn that at times, a complex subject might need to be "worked". This is not a waste--this is learning by exploring, by experimenting, and by trying more than one, single approach.
Learn what your software can do for you. Learn the curves tool first of all. It might be the most powerful of all the editing tools.
One of the real keys to being able to shoot photos well is simply
practicing how to do it well.