Hi K.,
I have 3 kids as well, and have been using an au pair for the past three years (my kids are 3, 7 and 10) - and now that I have had one, I probably won't go back. I have had a live in nanny for a year prior to the au pairs (domestic) and then did day care for almost 5 years before that... I think I have tried it all!
So - overall I checked out most agencies, but settled with Cultural Care Au Pair to use. They have a 1:1 matching program that was super important to me. I adopted all of my children, and in the adoption process I learned that some agencies will "pit" adoptive families against each other, and you would get into a bidding war for a child, just like with a house. I found it horrid in adoption, and changed agencies... when it came to au pair agencies I didn't want to go through it again.
I've been pretty lucky. I have had 3 great au pairs in 3 years, and in a few weeks will welcome my 4th au pair from my home. The transition wasn't too bad, but I had my college age sister living with me in the past. I am pretty open to having someone in the home, and have found that I enjoy having young adults in the home and joining our family. Maybe it's the adoption thing, but we are open to our family having people join it... :-) I found the biggest issue with the transition is being willing to communicate. For the first month or so of an au pair at home with us, I spend a lot of time - talking, making them feel welcome, explaining my rules, setting limits, talking more, and just being an au pair.
On the positive side, I needed the help - and I have so much better time with my kids when I'm not working. I can focus on them when I'm home, instead of worrying about if the house is clean or if they have clean underwear. The au pair takes care of so much, and gives fantastic care and attention to my girls, that I will probably have one for at least another 2-3 years. Good luck to you!
L. in Seattle