In good light the three phones, quite predictably in fact, capture similar levels of detail - that's not where you'll observe any major developments between these three generations. Having said that, the 5T's shots are that extra bit softer than the other two. We reckon it's a detail vs. noise reduction conundrum and OP leaned towards the latter on the 5T.
We find the 3T's photos to be the noisiest, while the 6T's are both the cleanest and the most detailed - that better be the case, really.
More importantly, however, the 3T lags behind in dynamic range, quite significantly so. The multi-frame image stacking wasn't really a thing back then and it shows. In high-contrast scenes, of which we had plenty thanks to the overcast weather, the 3T is no match for its newer stablemates, both of which perform similarly.
We found the 6T to produce substantially punchier colors when it detects greenery, and it also warms up the foliage - the snail shot and the tree moss are prime examples. It's not that we're not liking it, strictly speaking, but it's just a bit excessive.
It also cranked up the reds beyond reason in the firetruck shot. We found that a bit surprising, given the relatively similar color rendition in the 'Visions of the Future' poster set above.
In low light, the 3T is showing its age too, though it's still performing alright for a two-year-old phone. It's capturing as much detail as the 5T, on occasion even better, but it's generally noisier. That said, the 5T turned out by far the noisiest in the second scene, go figure.
In challenging conditions such as these, the 6T is easily the best of the bunch, capturing detailed and clean night shots, with plenty of dynamic range and nice color saturation.
The latest model also has a Night mode - of course it does, all the cool kids have it. It'll maybe lift up the shadows slightly, but it comes at a significant detail penalty and we feel it's unnecessary given the already great low-light performance. When we reviewed the 6T, we found the Night mode to work better with tripod shots, but we figured we'd skip this part for this three-way comparison.