Purple isn't a color/ Neither is lavender or champagne. These are all just BYB terms for blue fawn.
As for a blue fawn tri being generically impossible that is incorrect. If it wasn't possible there were be no Isabella Dobermans or Dachunds, which there most certainly are.
You see, tricolor isn't a "color" but rather it's a PATTERN that acts on colors that have been predetermined by genes at other locations. As such you can have black tris, blue tris, red/chocolate tris & yes, even blue fawn tris.