co-third-cousin-in-law

Spouse's third-cousin's spouse.

My co-third-cousin-in-law is a good person.

co-third-cousin-once-removed-in-law

Spouse's third-cousin-once-removed's spouse.

My co-third-cousin-once-removed-in-law is a good person.

co-third-cousin-in-law

1- Spouse's third-cousin's spouse.
2- Co-3C-in-law.

My co-third-cousin-in-law is a good person.