female, 5ƌ" 142 pounds. Go 3 times week.

I don't know how hard you work out and how much exercise you get the rest of the week, but if you're like most of the rest of us, then 3 x a week will keep you stable.
If you want to lose weight, you need to go 5-6 days a week or do gym 3 x a week and something else like fast walking or running the other 2-3 days. Remember that you have to produce a 3600 calorie deficit to lose one pound.
If you go 3 x a week, do 20-30 mins cardio (treadmill), then do 100 crunches to start, then do weights (5 types of crunches, 20 each). Don't go 3 consecutive days. Weights should be done on alternating days.

I've found for myself that doing my cardio (treadmill, elliptical, stair machine) FIRST, then going to the weights gives me the best results, so I use them both, in that order.