3 tremolo lines

How can I get 3 tremolo lines on the last eight-note?

Untie it, put it in a different voice, add tremolos, then tie it.

Or switch to Engrave mode, select the last notehead then adjust the Single Stem Tremolo property.

Note that the above notation is correct in the sense of a constant tremolo. Two tremolo lines on an eighth note correspond to the same tremolo speed as three lines on a quarter note.

Agreed. Simplistically, the flag on the eighth note replaces one of the tremolo strokes.

Yes, but in this case the f.t. Indicates flutter tongue, which is never measured. (Same with unmeasured tremolos in general, which also often use 3 strokes).

I like dorico‘s smartness, but here at least for my music it’s more often not what I want, than what I want.
(but of course I know the cases where this makes absolute sense)