If your initial budget is less than #2K, it's not going to make much difference which brand you choose. These days, a $2K bike is like a ~$150 fishing rod. Not much to distinguish between them save for the components. At that range, you're probably looking at the entry level Shimano groupo which used to be Shimano 105 for a road bike. They may have changed the name and/or introduced a line or two since I was heavy into cycling.
If you're not doing hills, you'll only want two chain rings on the cranks, not three. I don't recommend going with just one - if that's offered. Even on flats and when you're starting out, you want the small chain ring in front so you don't put too much stress on your knees. You want to build the speed up gradually, then switch as your legs warm up.
What you're going to want to do is ride a bunch of different brands and frames in your price range... Steel, Aluminum, Carbon will likely be your choices.
Steel : These frames will be the heaviest but steel is valued for the overall comfort of the ride. Very responsive without being too harsh over bumps. I'm actually not certain you can even find a steel bike anymore these days.
Aluminum : These frames will be lighter than steel, more efficient at translating your pedaling motion to power. They're very responsive but also super harsh over bumps.
Carbon : The feel on these bikes varies by manufacturer. For the most part, you get the responsiveness of aluminum in an even lighter package and without any of the harshness. However, the frames also have a tendency to feel dead. All the vibration and bumps are absorbed by the frame material, but that also insulates you from any feel of the road. This feeling is what differs between manufacturers and you never know which one is right for you until you ride a bunch of different brands.
I ride titanium, but you're not going to get into titanium these days for less than $3k probably. This is the material Zander eventually decided upon and he loved his bike initially, but the road bike was just a fad to him. The bike now hangs in his garage unappreciated.
For the manufacturers you listed:
Trek is akin to Pure Fishing... brands under this umbrella include: Fisher, Klein, Bontrager, Lemond ... I'm sure there are more.
Canondale... as far as I know is still independent. They're specialty used to be aluminum frames. Highly respected, often a little overpriced because of the name.
Specialized... their motto is Innovate or Die. I'm biased here because they are headquarted here in Northern California and I have their racing brand, SWorks, in a road frame and a hard tail mountain bike frame. I used to own a full suspension frame by them too. My first mountain bike was a Specialized Stumpjumper. Still have it. It's an all steel bike that I put smooth tires on and use it to tool around the neighborhood and took it camping all the time when we did that with the kids.
My recommendation, again, is to visit several different bike shops and ride all the frames and brands you can without any preconceived notions. Even if the first bike you ride feels like "the one" right off the bat. Go ride two or three more to validate that feeling.
Zander was convinced there was no way he was going to spend that coin on the bike he bought. All my friends think that going in. Then they fall in love with the feel that one bike and it's game over.
It's all about frame material, geometry of the frame you're riding, and how all that agrees with your body.
Stay away from hybrid bikes. I had someone ask me about those early on in his bike shopping days. He asked, what's the best hybrid I can get. I told him there is no such thing because by definition that bike is a compromise. But some bike stores like to push them or at least used to.
Oh and lastly, if it comes down to it, spend your money on a good frame with inferior components. You can always upgrade the components bit by bit... new wheels here (biggest improvement), derailer there, shifters too... but the frame is the most important part. Again, in your price range, this may not apply, but just in case.
That should give you a good start.