Nursing question.

I have a female cat that has had kittens before, over 1 year ago, and she is currently allowing my 3, 3 week old, kittens to nurse on her, will she actually start lactating for them?

It's possible, but she is clearly enjoying it or she would discourage them by getting up or hissing.

Is this cat spayed? And do the kittens have a mom?

The cat is not spayed,she just stopped having kittens. The kittens do have a mom, however I have been having to supplement them, as she doesn't get in with them often. The mom doesn't mind the other cat, she still nurses with her in the box.
The kittens and the cat that is not their mother.

Are this other cats nipples protruding?

She may just be a very motherly cat. Some are. How long have the kittens been nursing on her?

I haven't checked her nipples yet, but they've been nursing for about 3 days

She may get milk soon. I'd be curious to know if you manage to check her mammary glands. Otherwise it's comfort suckling, which is fine. Does this cat lick the kittens too?

Are the kittens getting enough to eat otherwise from their mom and your supplementing?

She does lick them and I feed them as much as they will eat every 4-6 hours unless they didn't eat very much or mom is nursing them