What is different in the engines of these three cars that makes them suitable for their respective applications?
The first misconception you need to manage is, vehicles as you've described are purpose built to maximize the different function. In other words, you really do have to consider the transmissions, rear-differentials, and tires when looking at these. You cannot separate them and hope to have any kind of "real" answer.
That said, theoretically, you could have the same basic engine for each of your models and still accomplish the same task. This is because of gearing in both the transmission and rear differential (or transaxle if so equipped).
The biggest differences between the three engines is going to be the camshaft(s) and tuning. There is not enough room in this post to go into the differences, of which, I'm no expert, so couldn't give you a "concise" answer to the differences needed. Secondarily, you'll have differences in intake and exhaust manifolds. Different intake runners will give you differences in where torque will happen (when put in conjunction with camshaft pattern). The same goes with exhaust. Performance engines will use exhaust manifolds which will get rid of the exhaust gases easier, so the engine can spin higher.
How does the torque vary at low, medium and high rpms in these three different applications - 1. high gas mileage, 2. high tow capacity, 3. high speed performance?
1.) An engine which is being used to get good gas mileage is going to have a torque curve which will peak quickly with a medium range torque and maintain that torque over a long range (from say 2000-5000 rpm). 2.) An engine which is being used to tow, will have peak torque early and then drop off quickly. This is why diesel engines are used in trucks. They have massive torque down low and live below 2000 rpm. Torque is what's considered to get a vehicle moving. 3.) An engine which is used at the track is made to spin higher (attain higher rpm), where you're not as worried about torque down low, but will peak toward the upper end of the rpm range. This produces more horsepower --
hp = (rpm * torque)/5252 is the equation -- and horsepower is what keeps us going once we get moving.
What are the pros and cons of a V configuration over an Inline setup, assuming same displacement?
The major reason for the difference for a V style engine over an inline engine, is the V engine will be able to do its work in a smaller space (engine bay) than an inline engine will because it is physically shorter. Taking this out to its maximum, take a look at the W16 configuration which is used in supercars. Very expensive to build, but can fit into about the same space as a V8 can. Same idea of more cylinders in the same amount of space. An inline engine will normally have more torque, while a V configuration will usually be able to spin faster. Older inline engines had the problem of fuel distribution, where the end cylinders usually ran lean due to their distance from the carburetor. With the advent of electronic fuel injection, this problem has been pretty much eliminated (and thus a resurgence of I5 & I6 engines).
These are some general thoughts about the questions you asked, as there is much more involved with what you are asking, particularly with the vehicle itself. While you didn't want to include transmissions, gearing, tires, mass, or anything else, these are very important to how you want the vehicle to behave and it's intended uses.