At the ranges for which the Kel-Tec P3AT is intended, no sights are required. Just sight along the top of the slide at any distance up to about 15 yards.
If you like, you could highlight the tiny front sight with nail enamel ("nail polish"). That's what I do to all of my pistols, including my P3AT. It helps sometimes, but not always.
Clean the sight with Energene or alcohol. Carefully paint only the back of the sight (the part you see when you're sighting) with the whitest nail enamel you can find. Don't slop over the top or over the edges. Let it dry for an entire day.
Now go over the white enamel with a coat of the reddest color you can find. Let that dry for a day. Then re-coat it with the red enamel, and let that dry for another day.
In low- or no-light, the red sight is black. In good light, it's bright red and easy to see quickly.
I suggest nail enamel because it's the toughest paint you can buy, and because it sticks to almost anything.
When you buy it, expect some cute backchat from the lady at the sales counter. If she, too, is cute, make good use of your opportunity. Who knows: you may get lucky, and she'll let you paint her nails.