Taking a second wife

I am married for seven years and have two kids aged 5 years and one eleven months old daughter. Whenever I seek intercourse from my wife, she declines me nine out of ten times, because she is either busy, she is tired most of the times, she is not well and that she has to take care of the children (these are the reasons according to her) and I am left being frustrated and when this kind of reasons are given I cannot oppose it and ask her forcefully. Sometimes we have intercourse just once in many weeks. I do not know what to do about this situation because it creates lots of frustration sometimes. Please help me with this. Also my wife feels that two children are enough where as I do want another child. She has refused me and have warned me as well to go leave her if I want another child. What I can do is in this situation to convince her?