Tiling upto an existing tiled external corner

[beanz]

It wouldn't be a 3mm gap. It would be closer to the depth of the bead. The radius of the quadrant. At least 5/6mm maybe more.
Depending if the tiles to the bead are cut would determine what i would do. I think the key question is what sort of finish does your customer want? There's ways of doing it simple, easy etc. Will your customer be happy with a capped finish? Or do they want neat trim. After thoughts are always a pain aren't they!?

That'll teach me to not fully read the post lol! I was assuming flat trim, as that's the only trim I use.. Doh!

 

[AliGage]

I re-read it just incase cause that was my first thought, just to turn it round.