I know that more than one female leopard gecko can be housed together, as can a male and female (but you could get eggs that way, so be very careful of that!). Two mature males should never be housed together as they will fight.
But where as she is already 3 years old, I'm not sure how she would react to a friend. If you want to get another one, make sure the tank is big enough for two, but I'd have a back up tank if things didn't work out.
I would LOVE to see a picture of your little girl! I have a little leopard gecko of my own, his name is Blink and is 7 months old. His picture is attached.
Shadow & Gus (the kitties), Blink (the gecko), Draco, Satine, Damon, & Outback (the fishies) ~I'll be there for you - 'cause you're there for me too~
Be at peace, Erin, I will miss you and remember your sweet face always...