I assume you are trying to butt 3 lengths of trim; 2 horizontally on top of the tiles & one vertical on the outside corner. I did not want to use trim, particularly on the vertical corners; I had it in my previous kitchen & thought it looked a bit naff!
I chose a thin mosaic border for the top, a bit more expensive but it looks nice. A really neat vertical, external corner can be achieved using a diamond wheel tile cutter. These are a boon & really cheap now; even the budget DIY ones will produce excellent results as long as you don’t rush it.
Cut the back edge of the tiles at 45 degrees, leaving about ½ mm below the ceramic face to avoid it chipping & just butt together the external corners. Once you’ve got the cutter set up, cut enough tiles for the whole job to make sure they are all the same.