basic bike questions from a first timer doing a sprint tri

First off...Welcome to the BT and triathlons.

If you have a bike that works and that is what you have been training on then ride it for your sprint.

You will see lots of very expensive bikes and lots of not so expensive bikes.  Just make sure that what you are riding is working properly and fits you (and your wife)

Have fun!

i agree with having the frame checked.  You say that the front wheel is trashed on the Trek?  What about the wheels on the other two bikes?  Depending on how old the bikes are, they will either have 700c, 650c or 27" wheels.  650c wheels are obviously smaller.  A 700c wheel is only a few mm smaller then a 27" wheel, so just swap out a front wheel and see if it fits.  You might have to readjust the brake pads a little, but it will work just fine.

As for the other two.... First off, go with the frame that fits you.  What is wrong with the wheels?  Out of true? Broken spokes?  Most shops will true a wheel for about $20, or you can invest in a $9 spoke wrench and about 40 min of an afternoon and learn to true your own wheel.  I'd suggest starting on one of the spare wheels you don't plan on using, but you should be able to pick up the skill pretty quickly.

For the most part you can swap parts as along as they match brand and form.  So if the two old bikes both have shimano components and they are both 8 speed (or 9 speed, ect..) then you should have no issues swapping parts back and forth.  If they are old enough to have friction shifters, as in no indexing, then you should be able to swap almost regardless.  So I'd say try swapping the better parts over to the better fitting frame and see if it works.  The worst case senario is that you learn a little about bike mechanics and have to ride your Hybrid afterall, which really isn't that bad of a situation.