Is there a popover for adding a major third (e.g. an F# on top of a D, when there’s no # in the key sig)?
I can do ‘a3’ for augmented 3rd, but I have to think about it for a bit first. 
Sure. Shift-I, M3 gives you a true major 3rd, even if it’s not diatonic.
Ah, Bisto.
Of course. I’d written off M in my head because of m. 
Or hit Q and type/play the pitch you want.
Side note: I’ve mapped Q to open the Interval popover. I never use the Quord function anyways, and when I’m stacking, I tend to think in intervals instead of note names. Also, Q sits nice and close to the row of numbers. Q3, Enter, Q3, Enter, c’est la triad. YMMV.