My favorite length for smallish water is 7'6" to 7 ft. I have 3 weights between 6' and 9' but use the shorter lengths for really small water and dense vegetation. The 9' is mostly for float tube/canoe fishing so I can be a bit lazy with my backcast. Its a mix of glass and graphite, favorites at present are a 7'6" Blue Halo glass and a 7'9" TFO Finesse. Average fly size for me is a 12, I can go to a 10 if it is lightly dressed.

Obviously, I like 3 weights.

The 7 footer will give you some reach over different currents on the small streams. Those streams have the same current issues that larger streams do. It's all about scale.
I have a 7 foot 6" 3 weight for small streams and it's not been a problem with small streams. That being said I AM going to build a 7 foot 2 weight over the winter.

Another vote for the 7'6" - very very versatile length rod that works on a bunch of small water, but will still perform on larger streams if you're fishing close in. Most of my UL fishing is on small streams where long rods don't do well - like trying to manuever a school bus through an alley way, when what you need is a honda civic.

8' is another good over-all length - my favorite 3 weight rod ever was an 8' 2 piece IM6 that I fished the bejezuz out of until I stupidly broke it by not casing it on the ride home once. Stupid, stupid mistake. Smashed the tip in 3 places. And naturally, the maker of that rod was out of business. The 8 footer did well on all but the bushiest streams, but I did have to be cognizant of the rod tip on some of my favorite streams (low hanging tree limbs that mandated side arm casts) it did great nymphing and casting dries on bigger streams where mending was necessary.

Maybe you should build two three weights - a long one and a short one