I have 3. Ages 4 months, 2 years, and 3 years. I had them all by c-section and I'm 41.
For me, three has actually been easier than two and I'll tell you why. When I had two, I felt like I had to do everything by myself. With three, I gave myself permission to get some help. A lot of help. I use a grocery delivery service. I use childcare for all three when I go to the gym, church, anywhere that has it. My youngest go to daycare two mornings a week while the oldest is at preschool and I don't feel guilty for one second about it! I drop my oldest at hourly daycare when I need to (he loves it there -- it's a chain called Kidspark). I told my husband before we even tried for a third (his idea to have a third) what kind of help I would need -- very specifically. Like, "I will need you to feed the kids dinner and put them to bed on Thursday nights so I can leave the house to work (I work from home part time from home)."I will need every Sunday afternoon for myself unless we hire a sitter for all the kids and do something together. We have started hiring a sitter about once or twice a month and my husband and I took up golf together. Before that we never hired a sitter. We never did anything together without the kids. I had to consciously make my husband a priority. It's easy to let that slide with all those needy kids around. It still happens more than I care to admit. I also told him I would need to go out a couple of nights a month with girlfriends -- bookclub, whatever. I never went out with just the two kids. Of course, he gets out too. Divvy up the work ahead of time instead of when you're exhausted and overwhelmed. Be very specific.
So oddly enough I have much more of a life with three. I golf, go out with my friends, have a couple mornings to myself. I go to the gym, Weight Watchers. I am much happier and more patient with my kids and husband because of it.
You will need help after the c-section! Do you have a Mother-in-law or someone that can stay with you for a while? The longer the better, even if they drive you nuts. I had my mom and dad for 1 week prior and 2 post. Then my MIL and SIL came. You'll need people to take the other kids out, help put the kids in their car seats to get to doctor appts. for you and the baby, help cook, clean, allow you to nap, etc.
All the other juggling/behavior issues you've experienced already with two, so I won't get into that.
As far as the risks that come alone with age go, you have to decide that for yourself. I have been very fortunate. Three in a row, no health problems, no miscarriages. I thank God every day for that!
You will have crazy days, and days your are exhausted and frustrated, but I'm sure you have that already.
Good luck and feel free to email me directly if you have any questions about how to deal with specific issues. I wish I had known more moms with three under three to pick their brains for tips/advice. I'm happy to pass on what I've learned by trial and error.
BTW, my advice sort of sounds like I'm giving orders. Not meant that way. Just suggestions. I'm kind of distracted while writing this...