The foundation of NFP is communication between the spouses about family size and serious consideration of the current ability to have more children. This forum is not really the place to hash out marital differences. It sounds like you and your wife are having problems agreeing on family size for eternity. Meaning you think 3 kids are enough and she wants to have more. I think the true spirit of NFP is to evaluate whether NOW is the time to expand your family. It is a consideration on a month to month basis. Perhaps, looking at it this way will alleviate some of the stress and allow you both to change your position as the situation dictates.
I would also encourage you to subscribe to the Couple to Couple League's magazine. It is a pro-NFP, Catholic organization that deals with many issues (including marital) involved with NFP. There website is http://www.ccli.org
I'm sure there are many other Catholic resources available to explain the proper use of NFP which would help you and your wife to approach this very important area of your marriage as a team rather than in opposition. Here are 2 articles that you might find helpful:http://www.americancatholic.org/Newslet ... ac0707.asphttp://www.usccb.org/prolife/issues/nfp/cathteach.shtml
On the 2nd link see pages under "openness to life" and "responsible parenthood".
Lastly, in all things, pray. Pray for each other and with each other.