Pump to Measure?

My wife will be having our third child soon. We have failed breastfeeding the first two times due to an inadequate supply. Do we need to get an electric breast pump this time so we can measure our baby's intake? The first time we failed, the baby didn't lose much weight but the breastfeeding thing just didn't work out. The second time, we accidentally/temporarily starved our baby until the pediatrician told us the baby had lost too much weight. I was very upset about this since I had no way of telling if our second baby was getting what she needed or not.

If I hook a breast pump up to my wife as soon as our new baby is born, will we then be able to get the milk our baby needs? My fear is that our third (yet to come) baby will suck on her mom and "appear" to be getting food while she is all the time losing weight just like our second child did. I really don't want to starve our third child like we almost did our second. Also, I'm wondering how many days of pumping it will take for the milk to come in. I've heard it takes three days, but I can't imagine not feeding my child for three days!!!

Thanks,

Some Dad