both my husband and i were from families of three. in each of our cases, and many other we have noticed, working families have trouble dealing properly with more than two kids.
you said you work. have you ever thought about bad financial times, or loosing your husband, or if the child is not healthy? i have two kids, and it has been such a struggle to raise them well. i had my own beautiful home in country, i got to stay home with them, and still, even with much support, ect. it was difficult.
if you must have another child YOU are the only one who will pay the consequences of doing so for the rest of your life, (don't forget the kids you already have lives, either) but that said, that it is your own choice to make/bed to lay in, that having two beautiful children is just about everything a mom could hope for. and the road is long.
i have asked many mothers of many kids, and they all said they would do it over, but did say they wished they knew a few pitfalls, before they made the numbers so big.
if you already work, and the economy is so bad and not going to get better, its my opinion you are tampering with a good thing. each child is a 20 year, quarter of a million dollar project.
i hope you make the choice that is best for your family. best wishes, dani