I would second the recommendation to go to several smaller conduits.. A single 3" may be fine for now, but if you want to expand in the future, it's a lot easier if you have a spare conduit or two to work with.
I would run at least three conduits, and leave at least one as a spare.
I agree that cat6 is overkill.. cat5e for data, and cat3 for phone is more than adequate. I tend to put at least two data jacks in each location, or 4 if I anticipate having two connections initially.
I agree with Ev about piping everything. it's worth it in the long run. Even if you don't anticipate putting a jack in that location, put a box and pipe so you can add one later.
I disagree with Hal on the coax patch panel. It's your house, and you have the right to make it conform to your standards. Do it right, use proper compression (snap-n-seal type) connectors, and make it clean. Any cable guy who whacks ends off my properly terminated cables is going to get a nasty call to his boss, and a bill for damages.
In fact, don't let the cable guy work on your wiring. Have him bring in his drop, and mount it where you specify, and take it from there. The cable guy will take the quickest route, and won't care how much of a mess he leaves you, how much space he takes, or how much of a pain he makes your life after he leaves.
Keep the phone, data, and coax terminal separate. It helps keep the overall clutter down.
Use a proper backboard, not one of these all-in-one panels that the builders are fond of, but that you will hate as soon as you try to work in it.
