A trike in many cases is a two-wheeled machine that has been modified with a kit that coverts the machine to a three wheeler. In these cases the insurance will typically list the parts under accessories coverage. Much the same as you would list upgraded handlebars or a new exhaust. The trike conversion kit may have a price of $10,000 or more, not to mention labor to complete the conversion. Another example of a three-wheeled machine are the newer Harley Davidson Trikes. These are factory-built trikes and will not need to have the policy reflect additional parts as it is not a conversion.
The most significant error made is to not list the converted cycle as a three-wheel machine, and to not list the parts on the insurance policy. When you are preparing to insure a heavily customized machine such as a cycle that has a trike conversion kit be sure that you have your receipts and current photographs. Take photos at each stage of the conversion if possible. If the kit used is a known manufacturer be sure to collect documentation on the company. All of this will make a claim go much easier should you ever have to file one.