You’re almost there! “Designed at” is exactly what you’re thinking — 1 virtual point on the screen = 1px at 1x, 2px at 2x, and 3px at 3x. In your case above, what you’ll want to do is bring your bitmap in at 900x900 and resize the image to 300x300 so that when you export, you’ll get a full res 900x900 image at 3x. Hope that helps!