"Scotch Ale. A brown beer rather resembling Burton. In the London pubs the term almost invariably stands for Youngers Scotch Ale, in bottle or on draught, which is a genuine Scottish brew. As this is a very popular drink it is often found in free houses, where it usually replaces a Burton, though there are a few pubs that sell both. Younger's Scotch Ale is their No.3. Their No.1 is a really strong brew."
"The Younger Centuries", by David Keir, 1951, page 88.
So that explains what No.3 was: a Burton-like Strong Ale. I drank the stuff many times and always wondered what the hell type of beer it was. I seem to remember it occasionally being called a "dark Bitter", whatever that means.
Here are some details of Younger's Scotch Ales:
That's me about done with Scotch Ale. For the time being, at least. I wonder what will attract my attention next?