tnal

At the moment i have a linc town car that im upgrading the sound from factory JBL. I have two dash speakers which will be 4" polks 2 front door speakers 51/4" cliff design components and 2 alpine 3 ways in the back deck. I have a 4 channel 50 watt amp and a 2 channel 50 watt amp down the road i might also ad a sub. My question is what do i need to make this work? All the 3way elect crossovers use the third channel for a sub. Any ideas?