As justbill says, there are 4 pairs of pins on the lower left for the (up to) 4 CO lines in the 551C. Since you have 3 cards, you can wire up to 3 CO lines.

If you are going to wire up the intercom, I might suggest you wire the 2564 keyets directly to the 66 blocks, then run cross connects from the 551C to the 66 blocks. You can pull the 24VDC battery (talk) and the 10VAC intercom buzzer voltage (for the intercom) from the right lower side of the 551C and cross connect it out to the 66 blocks.

On my install I ran one set of cross connects out from the 551C to the first phone on the 66 blocks, then just daisy chain with bridging clips and more cross connects. That reduces the clutter on the 551C internal block itself, and you don't have to go back in there if you add more phones.

There is a large brass ground screw connection inside the 551C, better hook this up to a solid earth ground with a #10AWG solid conductor, otherwise the line cards may not pick up incoming calls.

Hope this helps, -Stryder