I can't attest to being able to fit 3 crates in a Cirrus, but as far as removing the rear seats, that's easy
remove the seat bottom cushion (held to the seat frame by velcro), then find the hitch pin style hardware holding the seat bottoms to the airframe (2 per seat, near the forward corners of the seat). lift the forward edge of the seat bottom up about 2 inches, then slide the seat assembly forward to disengage the 2 pins holding the aft edge of seat to the airframe. if you need help, have a friendly A&P take a few minutes to show you, as a "picture" (or seeing it in front of you) is worth a thousand words!
unfortunately, the structure under the rear seats is not level with the baggage compartment floor- it slopes down toward the rear of the airplane. something i found interesting is that apparently there are other cirrus owners that want a larger flat area with the seats out, and we have something like this available: http://cirrus.tamarackaero.com/cargo-conversion-2/