I've been going back and forth on what to do with these midrange speakers I bought. I had planned to build a 3 way component set. I have a memphis m class 6.5 with focal 1" tweeter and a focal cross over I was going to use up front. But I'm thinking of mounting the closed back mid range on the door with a mounting ring. Running the comp set in factory locations with crossover off the front channel and midrange off the rear channel with a rca split from the front. It's fq response is 300hz-8khz I beleive without going out to look. Using the amps cross over to set it at 300hz, and running the comp set off the front channel crossed at 80, and lastly the sub (vfl 8) crossed at 100. Leaving the rear doors alone for now. Would this cause any problems, or sound alright? Not worried about the door panel it's self. Just how it would sound with all three speakers up front. Pic for speaker position reference.

It will sound worse than just running the 2 way you have.

The whole benefit of having a 3 way is getting to use a single driver optimized for the great majority of the musical range without giving up midbass or top end sparkle.  You will gain non of that.  The two most important steps in deciding on a logical 3 way setup are 1) driver choice and 2) setting up the crossover appropriately.  The 2nd one either requires some serious savvy in making passive crossovers after measuring your speakers in place or a flexible processor with the capability of providing the crossover portion electronically.