freakinout - I'm in the same boat too, am now 26w with dc3 - totally unplanned, in fact I was adamant that dc2 would be my last even though dh kept going on about having 'just one more'....
I cried for the first few days after finding out, because I too kept thinking that life would jst be starting to get easier ds is 4.6, dd is 19m - so will have 3 under 5 for a while.
At my 17 week scan there was a potetial problem flagged up which terrified me and made me realise how much I did want this baby actually, and couldn't bear the thought of not having him. All was fine, thank goodness, but it really brought it home to me.
I hate being pg too, which doesn't help things, but this pregnancy seems to be going in much faster than the last two, actually am starting to panic that I am completely unorganised for this baby! I'm also overseas with nobody to help out, but my mum and mil are both coming over when baby's due to help out which will be good.
Anyway, what I really should be saying is that I am now really looking forward to meeting this child, can't believe I'm going to have 3 kids! And I know it will be tough, and tiring, and I won't have a moment to myself, but what the hell, how much harder can it really be? (naive hopeful emoticon required)...
Give yourself time to adjust, you'll come around to the idea, I promise!