Hi Carson,
Firstly a 5/6 weight rod would, arguably, be considered ‘light' by a large percentage of anglers but I would consider this a good all-rounder for stillwaters (not knowing where or the methods you intend to fish).
I have no doubt that the 3 wt can handle any 'stillwater' fish that you are likely to encounter and would be an excellent choice for the Cowie and Carron. It would have its' limitations when bank fishing a stillwater where distance required, wind and fly size/type may be an issue.
This is where the slightly heavier weight would be an advantage.........I thought that I'd found my perfect rod in a 9' 5wt Guideline but that has been pushed, temporarily maybe, aside by the 9' 4wt version which think is a fantastic all-rounder - I used it in that howling conditions yesterday at Lochter and must have cast a lonely shadow casting and stepping along the down-wind bank but picked up 9 fish just yards from the bank.
it's much more rewarding to fish a lighter outfit at shorter distances and use the head than to follow the bicept brigade
By the way I use the Barrio 3wt for my river trouting and it is a pleasure to use - give it a try.
Happy hunting
Iain