I've always wanted 2 kids, and now I have 2 girls.
I definitely would need to quit my job during the pregnancy if I choose to have a third. I can't do the working and parenting while pregnant thing again. Much less the baby phase while working and parenting 2 other kids. I was so exhausted the last time and there was no reprieve. Pregnant and childbirth also were difficult for me, and I would probably go for an elective c-section if I had to give birth again. I just wish I could get it out of my head.
Sooooo, is it really baby fever that will pass? Or is your brain telling you that perhaps your pre-kids decision that you want 2 kids isn't true now that you actually have two? You're listing all the problems with having a third, but other than the inherent difficulty of a pregnancy and infant, are those really problems, and are they worth not having a third over? If you can't work and parent, what would be wrong with quitting your job? Many do. And a C-section isn't awesome, but it's available if you want it. Or look into adoption if the pregnancy itself is keeping you from something you want.
Now, you may very well decide that you really really want to keep your job more than you want to have a third. But you need to make that decision based on who and where you are right now, not based on a vision of what your life was going to be like that you developed before you had any life experience at all. At least IME, that kind of recurring pang means that there's some sort of emotional need that I've been ignoring. And the more I try to ignore it or tamp it down -- the more I tell myself that I
shouldn't want that -- the more it insists on popping up (usually at incredibly inconvenient times).
The only way to make that emotional need less powerful is to bring it out in the open and put it on the table with all the other options, and weigh it fairly. Acknowledge that that need exists, that it is legitimate, and that you can make it happen if you decide that's what you really want. Then work through what you'd need to do, what your life would be like, if you did make it happen -- giving up the job or getting a fully-time nanny, sleep deprivation, etc. -- basically, play the "what if?" game. Then do the same thing with your other options (here, sticking with 2). Once you have your real options all laid out before you, you can make a better decision that is easier to live with.
IOW, there's a difference between telling yourself "I shouldn't want this, make that feeling go away" vs. saying "I do want this, but if I go after it, that means giving up ABC, and I value ABC more."
Also, if the youngest is only 8 months, you don't need to make a decision just yet! It would be entirely reasonable to table this decision until your youngest is at least sleeping through the night! That really gives you a third option here: (1) have #3; (2) stick with 2; or (3) decide not to decide just yet and come back to it later. Even 8 months out, your hormones are probably still going haywire, and the sleep deprivation is chronic. So you very well may feel different and/or have more clarity in another 6 months or a year.