I have pulled our Cougar 1 trailer with a cyclocross bike (road-geometry with beefed up frame/wheelset) and with a hybrid. Both are okay, but the mount is via a skewer. I don't think I'd put a chainstay mount on a road-bike. The gearing comments are appropriate if you are planning a road-double, but I think I heard you say road-triple so no worries. One suggestion though, if you can find it get a 30/39/52 instead of a 30/42/52. 39/25 is just so much easier than 42/25. Anyway, here are my observations. Trailer + Child ~ 50 lbs. Me ~ 220 lbs.

Cyclocross bike: Kona JTS. Gearing: 28-39-50 Chainring, 11/32 9-sp cassette.
No problems towing around town, but my mirror is a bar-end mirror and I can't see both my daughter and behind me, its either or. As soon as the grade increases to 6% or so I am in or around 39/28. For longer(1/2 mile)/steeper(>6%) uphills, I have hit 28/32 several times. For downhills, I don't think the brakes are strong enough, and I get significant pushback from the trailer. My typical comfortable crusing speed with the child is about 12 mph. Without I go around 16. Biggest issue: no kickstand. I have to lay the bike down to take the child out of the trailer because its too hard to find a wall/object to prop against and still be able to get a the trailer. Plus, the trailer moves the bike a bit as we take our daughter out, and the bike tends to fall even if propped up. I can't fit a kickstand to the bike because of clearence with the bottom-pull derailleur and the small space between the chainstay bridge and the seat-tube.

Hybrid: Specialized Crossroads Elite XC. 28/38/48, 8-sp 11/32.
The upright position and bar-end mtn mirricycle allows me to see both the kid in the trailer and traffic behind. The upright position doesn't rob me of any speed unless the wind is REALLY blowing. The linear pull brakes are much better on downhills than the cantis on the JTS. It feels a bit more natural to me to be pulling the trailer with the hybrid than pulling it with the JTS. Gearing is similar to the other bike, but I find myself dropping to the 28 ring more often on this bike than the other. I don't have a computer on this bike, but I think I probably go slower on it, mostly because of the attitude/feel of the bike. The kickstand makes this bike much easier to use since I can stop in the park right next to the play area and let our daughter run free much more quickly than I can with the JTS.

I don't want to discourage you from getting the type of bike that you want, but definately concern yourself with quality brakes/pads and proper gearing if you go road-bike. Also concern yourself with the ability to mount a kickstand. Road-bikes often have tight clearence between the seat tube and the chainstay bridge with bottom-pull derailleurs. That means traditional kickstands are likely out of the question, and chainstay/hub mount kickstands may interfere with the trailer mount. I hope that was helpful.