Best situation will be you'll meet your future wife.
Worst situation will be she will say "OMFG no way, gross!" or it's actually the wife of one of your colleagues and you get fired.
Most likely situation, she gave you her number because she was willing to get to know you better. Invite one over, or both. This is a no brainer.
Fuck the 3 days rule as well. Did you like them? Did you enjoy their company?
"hey it's Marty from last night. It was lovely speaking to you, I had fun

. Want to *suggest something fun, but not so fun that it takes over and you can't get to know each other, also make it non threatening, and obviously a common interest* on *insert a day*?" - maybe personalize it with an inside joke you guys had?
Simple. If you're not ugly and not a mong, that should work. It's never failed. Fuck what they think. Send it in the evening the day after meeting them. Don't even fuck around questioning your self. Just do it. What's the WORST that can happen?