Carrier 326600-751 Heat Exchanger Assembly

Does this include the instructions for replacement or are they available online anywhere?

Unfortunately factory didn't provide us any instruction document as these are intended to be replaced by a licensed contractor.

I have three cells in my heat exchanger assembly in my furnace. Do I have to buy three of the 326600-751?

Yes this part number is for 1 pc as shown in the picture and if you are replacing 3 pcs then you need 3 pcs. If you are not sure please reply with the unit model and serial numbers so we can check with factory. Thank you.