I've set the relationship between the two valve station fields as equal and allow the creation
That doesn't really tell us enough about your relationships. What fields are matched with that = operator? How do those fields specified as match fields get their value?
It would appear that you need this relationship:
Table2::__pkValveID = Table3::_fkValveID
where __pkValveID is an auto-entered serial number field and _fkValveID is a number field (no auto-enter specified).
Then, to create a new record in Table 3, you either select a value in _fkValveID that links it to the correct Table2 record using a drop down list, or you set up a portal to Table3 on Table2 and enter data directly into a blank row of Table3. (Then the "allow creation..." option causes FileMaker to copy the value from __pkValveID into _fkValveID of the new record for you.)