Why does InnoDB table by default uses the UNIQUE constraint instead of the PRIMARY KEY?
With InnoDB tables, all secondary indexes include the columns of the clustered index (which is the primary key), appended in the end. So your unique index has actually 4 columns, the 3 you have defined plus the 1 primary key column. 

When running a query that needs a full table scan, both indexes have all the data needed, so the optimizer is free to choose any one of the two indexes. In fact, as explained in InnoDB Table and Index Structures, the clustered index contains some additional information that costs 13 more bytes per row:


  Records in the clustered index contain fields for all user-defined columns. In addition, there is a 6-byte transaction ID field and a 7-byte roll pointer field. 


This makes your unique index less wide than the primary key index, which is why it is chosen by the optimizer.