What makes you think you need a 3PST switch? Here's what you need:
**broken link removed**
As someone here likes to say (one of the Rons?), the diode is your friend. They protect the normal independent operation of each light. When you flip the switch, all 3 lights will be illuminated. (The switch can also be a flasher circuit if you want flashing hazard lights.)
The dashed lines show the existing connection to each light.
By the way, I think the words you are looking for are "brake" and "sense".
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Whoops, my bad.
Do not use 1N4007 diodes! Your house will burn down! Your cat will be electrocuted! Your hair will catch on fire!
Heh, just kidding. But I missed your "10 to 15 amps" up above. (Do your lights really draw this much?) In which case you'll need to use much beefier diodes. I'd suggest something like
this diode from Digi-Key. It's a Schottky diode, which you definitely don't need, but no harm, and it's cheaper than some other non-Schottkys ($1.88.ea), and can handle 20A, which should safely work for you.
Actually,
this diode would be better (a little more expensive, $2.60, but can handle 35V reverse voltage instead of the 15 of the one above).