Cage length doesn't have anything to do with BB width or chain line. Cage length just determines how much gear range you can handle. If weight isn't a problem there is no reason not to keep the longer cage. Even though it isn't advisable to cross your chain to the extremes (large sprocket to large or small to small), it is good to be able to do so. That way if you accidentally down shift too far while still on the large chain ring, you don't twist your derailleur off. Also when climbing a hill you may need to downshift for that last 100 feet and may not want to shift to the small chain ring and then up shift the rear a gear or 2. For a 100 feet once in a while it won't hurt. I have a 10 speed compact crank with almost as much range as a triple and I use a medium cage derailleur. Your requirements will depend on your total gear range.
As for chain line you may be OK. Ideally your chain line should be correct, but if it's a little off to one side you will have more chain wear when crossing against the chain line. If your crank is too close to the BB you may also drag the chain on the large chain ring when crossing from small to small sprockets, it may do this even with a correct chain line. In your case the double crank will probably be too far out and that won't be a problem. The good news is that you will have less chain wear when you don't cross (lol). You will probably be off a couple of mm. See Sheldon Browns page on chain line: http://sheldonbrown.com/chainline.html
. If you don't often cross to far against the chain line or can live with the added chain wear, I wouldn't worry too much. Otherwise get a new BB, unless you have something special they aren't that expensive.
Your front derailleur adjustment will depend on the length of your adjusting screws. It shouldn't be a problem, depending on the specific derailleur. A bigger problem may be in your 3 speed shifter (I'm assuming you have some sort of indexed shifting). My Ulterga 10 speed double STI shifter has 1 half click, A friend's 9 speed shifter has 2 half clicks and I suspect it may really be a triple shifter, however it seems to work on his double crank.
I suggest that you try to adjust your front derailleur on your triple crank to just use the 2 larger chain rings. This will be similar to having a double crank and should give you the information you need. You will probably run into any problems that you will have going to a double.