When establishing relatioh-ships between two tables, and more than one key field is used, is it better to enter all the key relationships in the two tables, or , using duplicate tables, use each dyad to link by single key field?

Example
Linking  between two tables , using Surname, First Name and Home Telephone
Am I wiser to  have Table A1related to Table B1 via Surname and First Name
And then show the relationship between Home Telephones in Table A2 and B2?

What happens if I link Surname, First Name and Home Telephone (all three key fields ) between one iteration of each table Table A and Table B and two of the three key fields match but a third does not ?
Will the portal in Table A still show the related data from Table B even though  the Home Telephone numbers do not match between Table A and Table B?

I guess what I am asking is whether Filemaker understands the selection of several Key field relationships as implying 
First Name AND Last Name AND Telephone to see the relationship?
Or will Filemaker see the relationship  if ANY of the Key fields match ?

I have never seen any explanation of this.