How to handle the sale of just part of a bundled depreciable asset?

My CPA 7 years ago bundled 3 pieces of equipment (trailer, UTV, and a chipper) into one depreciable asset bundle valued at $13k with a 7 yr life.  It seemed odd, but also looks to be something that is allowable.  Now 7 years later, I sold the trailer for $2k (original price $3500).  How do I report this?  Do I retroactively split the bundled depreciable asset into 3 pieces (basically, enter them all in and delete the original), then document the sale of just the trailer, or is there another way to do this?