I know the urge to have a third one when you've got two kids is immense (when the youngest one gets to sort of 10 to 12 years old.) I've had it myself (I have 2 girls less than 2 years apart.) I was in my early 40s at the time and I even came off the pill and tried. (DH wanted to too!) It never happened. I was disappointed for a year or two, but I am so glad now. (So is DH.)
I'd have been 43 or 44 when the third one was born. I can't believe I even thought of doing it. This was like, 15 years ago, and I would have had a junior school age child at 50, and a child in their early teens now when I'm knocking the door of 60. It fills me with absolute horrors just thinking about it. 
Since 49-50 y.o. (nearly a decade,) me and DH have had an empty nest, as our 2 DDs left home in quick succession, one year after the other and never came back (both went to uni.) We both love our girls dearly, and enjoyed every second of raising them, but that part of our life is done, and I can't imagine why we thought about starting over with another one when they were virtually teenagers, and almost independent.
Having an empty nest at 50 was wonderful. Having a full one for 20 years was wonderful, but having an empty one at 50 and not having to worry about young/school age children anymore is lovely! We now have 2 intelligent, successful, brilliant young women - with their own home - who are now our adult daughters - and our best friends.
I know you're a bit younger but you'll still be in your early 50s, with a child in their early to mid teens. Instead of being early 40s with a child in the early to mid teens. Honestly how much more tired and weary you feel, and how many more health problems you have in your early 50s, compared to early 40s, you can't even imagine.
I see a few people on here thinking having a baby at 41-44ish, and they think it's great idea because they feel really great and healthy and young - but don't underestimate what the menopause does, and how your health can change in that one decade - early 40s to early 50s...
Also, unless you're actually very well off, on six figures, have your home paid for, and have a few hundred thousand pounds put away, don't do it. It's going to be hard enough with two as they grow up - with school, college, university, trying to help them financially. We struggled financially for quite a years, and only came out of struggling properly when they left uni as we are not on massive salaries, just average like most people. Don't put more struggling into the mix. Stick with two children. It's a perfect number of children. (So is one!)