Its all going to depend on your size BEFORE the babies, and you're body shape now. you can def. tighten up tho. 3 sets of 30 a day is fine, you really can't OVERWORK the pc or kegel muscles.
How long have you been using your KM? what setting are you on?
Also, its not so much a matter of being 'tighter' but having the muscle control to 'tighten up' when you want to... that is one of the big keys with the sexual benefits. If you can control the squeeze, you can keep tightening and strengthening so that you can actually HOLD your husbands penis inside your body.
Hope that helps... let me know...