How can I make a custom report that displays info for three contacts based on relationships?
When starting on reporting I did a lot of debugging in my report and in the parent class to find out what was happening.

Your specific error: I strongly suspect 'relationship_type_c_civireport.label' should be 'relationship_type_id_c....' but I am not sure.

Check if there is a postProcess function in your class. If there is, it should call the parent buildQuery function to build the Query, you can check here how the query is build step by step. 

Also, the select part of the query is build in the select function from $this-&gt;_columns. If you debug the values of $this-&gt;_columns you should be able to find your problem. Happy hunting! And keep going, it is hard work at first but you will get there.