Wiring Component Speakers

You still would be defeating the purpose of a three way system with a 6.5"

A decent 6.5" mid-bass  generally can be played down to about 80 hz with some sucess, but ideally an 8" woofer is the best way to go. You will be able to fill the gap between your sub woofers and midbass that can occur.

I am sure DYohn will give you a further explanation here.

I agree.  Most standard 6.5" car audio speakers will not gain you enough over your 5.25" to be worth it, although there is one 6.5" that I know about that plays with authority as low as most 10's, and that's Adire's Extremis 6.8.  Right now it is only available as an 8-ohm version (which is fine for car audio as long as you give it an amp that will deliver 100 watts into 8-ohms) but rumor has it there will be a 4-ohm car audio version avaiable soon.  There are other killer mid-woofers available from companies like Dynaudio, Focal, ScanSpeak, Morel, Seas, and others in the 7" to 8" range.

As dwarren suggested, I am steering you away from using multiple speakers playing the same frequencies and towards converting your 2-way components into 3-way components.  What you want to end up with is something like a subwoofer crossed over to play below 80Hz, a mid-woofer crossed between 80Hz and about 200Hz, and then your 2-way comps playing everything above 200Hz.  This will probably require an electronic crossover, or a second passive crossover for the mid-woofers, and more amps than I believe you currently have in mind.

Let us know how you decide to proceed and we can help you set it up.

Now do you see why I at first suggested you may want to go find a pro in your area to help you?