Have 2 kids, wife wants another, I feel I'm at my limit. Advice?

Long story short. I'm 35, my wife is 36. We have two kids. 3 Yr old boy (born premie at 34 weeks but doing great), and an 18 month old girl (born premie at 30 weeks and has some development delays and issues). I absolutely love being a Dad and my kids bring me so much joy. However i have become quite a stressful person since we've had kids and so has my wife. I barely struggle to handle the workload of two kids let alone thinking of a third. My job (I run a business so can't walk away from it so please don't suggest it) is very stressful at times and drains me a lot right now as I'm fighting to take the business to the next level. We have financial stresses in our marriage and money is tight and the idea of my wife taking another 18 mo ths off or so to have a third means we go down to one wage again which will be very difficult.However, my wife is super keen on a third and doesn't feel like our family is complete. I want to give her that and I feel I'm being selfish by saying no, but I am utterly overwhlemed and feel like having a third will give me a breakdown (I don't mean that as a joke). I've explained to my wife that I am overwhlemed by all this and this is a bug decision and she understands that. However I don't want her to resent me in the future that I took her dream away from her.For those who have 2 that decided to keep it that, do you regret it? Why did you stop at two? For those that went on to have three, how did that go? How did you cope? Is it a big step up? How did you manage!? Did you have to get a bigger house, bigger car, bigger debt lol??Any advice would be great as I'm a bit lost. Thanks. via /r/Parenting http://bit.ly/2RY2br1