normally when in regards to "3-way speakers" they are also coaxial speakers...as stated tho a component set is generally your mid bass woofer and a separate tweet but there are sets that will come with more speakers...the infinitys will sound 100 times better when if all you got was coax all the way around. just do yourself a favor and put at least some sound deadening around the speaker mount.Front speakers are components or three way speakers(mids and lows in one speaker with your separate tweeter giving you the highs). The rear doors are coaxials or two way speakers (highs and mids in one speaker). They are the same size though. The front and rear door speakers are 6.5". This explanation is a little over simplified, but gets the point across I think.
Component vs Coaxial
no back to your question you can run the front 6.5" woofer in the rear...you most likely wont notice a difference with missing the tweet due too how loud/bright the fronts will now be....and most audio set ups your highs are in front