That's an interesting question, and one that may be slightly controversial, with there being at least small amounts of stocked fish in the ones mentioned, i realise that Loch Leven have reverted to a natural brown trout fishery program, Lochy has been known to produce rainbows, even Loch Tay has some huge escapee rainbows (albeit not both stocked intentionally).
You may find that most people, (myself included) have primarily fished the Rivers Tay or Lochy for Salmon and on such days, landing a brown could be construed as a nuisance (on hindsight though) that was certainly the wrong way of thinking.
Sadly, on the Lochy at least, Brown Trout are regarded as predators of the smolt program that was undetaken by the restoration program near Glenfinnan and was run with the aid of Marine Harvest.
I always suspected for real wild brown trout fishing (rightly or wrongly) one had to head further afield possibly as far as Sutherland Orkney, shetlands etc.
I don't believe i can honestly say, hand on heart i am aware of fishing three really productive wild brown trout venues, although there are a few not too far from me, i don't feel they are what one may describe as 'prime' fishing destinations, however that said it will be interesting to read if there are some venues closer to home than i thought.
Good question diehard81