I'm no expert, but based on what I've read, 3 sessions equalling 45 minutes is fine! Just be sure you're putting in an effort when you do them. And you're smart not to use it when your baby is apt to get too near. A woman I used to work with had her dd lose part of finger from putting it into the wheel.
I don't know much about home weight machines, but based on common sense, I'd say aim for sets/reps. That way you'll do what you set out to, whereas if you say x minutes, you may not get as much benefit.