Start with a simple white coat. This will help bring out your three colours
Put your first colour down, preferably your lightest one. It doesn't matter how messy you are, for this the messier the Better! You just want strokes or blobs all over your nail
Just like you did in the last step, put the next colour down, preferably the "medium" one
Next put down your last and darkest colour. This is the end of the stroking stage so if you have any unwanted white spots you can go in and touch them up with more of your three colours.
You can add a light sparkle polish over this. (Can't see mine that well in the photo) but you don't have to. For the next step you want these coats to be completely dry!!!!
Next, cut a triangle out of tape and stick the tape on your nail with the triangle in the the middle. And if you have the thin nail tape you can use that to make a smaller triangle on the inside
Paint over the tape with black nail polish
Remove the tape as soon as possible after you paint over it with the black polish. If you wait the black will pull off with the tape.
Then clean the edges, add some top coat and your done! You can fool around with the way you use the tape/triangles to make unique designs!