First, it could actually be drool. This is gross, but you have to really smell it to tell - I know that my dog drools quite a bit and I've thought that he had peed on his blanket. So I'd watch for the next few days to see if it continued.
Second, my dog, when he was a puppy, did pee in his sleep. NO ONE told me, and nothing I read, ever mentioned this - I figured it out on my own when I kept discovering him in his crate at night, covered in pee! And one time he was asleep in my lap, and bingo, he peed on me. He was like a little kid who wets the bed - the impulse that wakes people up to pee in the night takes a little while to develop. My puppy slept so deeply that he would pee in his sleep, and THEN wake up and bark because he was now cold and wet. It went away as he matured.
Since your dog is 3, I would see if it continues over the next day or so, and confirm that it's not drool. Then definitely take her to the vet - you don't want a simple bladder infection to get worse.