Last tested: Nov 25, 2019
Yes. If your connection test returns an issue with accessing the connection_reg_r3 table or your PDTs fail to build because the regenerator is unable to access the connection_reg_r3 table, you CAN safely drop this table.
Dropping this table will not have any affects on existing PDTs. The connection_reg_r3 table will be recreated the next time it is needed (i.e. when running the connection test, when the regenerator runs). Any connections that were registered in the connection_reg_r3 table will have the same registration key in the new version of the table.
One thing to keep in mind is if you are deleting this table because the looker user cannot access it, you may also have issues with the looker user accessing the schema itself/ PDTs on the schema. If this is the case, you will need to modify the looker users permissions as specified in our database configuration documentation.