Depends on the existing triple speaker set up, are they OEM or aftermarket - brand?
It seems odd to add an external crossover to OEM speakers - suggests aftermarket stuff - picture of the crossover?
Basic audio is tweeter/woofer, the uprated OEM set up added a mid speaker - all the door cards are prepared to accept this additional speaker, a PO could have retrofitted OEM stuff or even uprated the speakers - if they are aftermarket I would consider keeping them.. but depends on what your planning to install.