Hi there,
One thing I think you need to consider is your vehicle when making this decision. I have three children, 7, 5 & a 11 month old. I was going to trade in my minivan last year during the early part of the summer but found out I was pregnant with #3. So I had still contemplated getting a sedan but realized that you can't fit three car seats in the back. I needed two boosters & then an infant seat & no sedan that we looked at would fit all three.
The other thing is do you have room in your house? Having a baby you have all the baby items all over the house (ie: swing, pack-n-play, high chair). Don't get me wrong I was exstatic that I was having a third one but our home is very small. So we had to cut down on what we kept & what was passed on.
Daycare expense is another thing to consider, who will care for the baby when you go back to work? Who will watch all three kids when school is out for the summer? The time when there is no school or a delay? If you work from home then great, but if not then that creates a new problem.
I don't think that age would be a problem in having your third. These are just the few that I could immediately think of when I read your post. Good luck with your endeavor & I hope it turns out well for you.
M.