To check for the marriage I need to know where both Michael and Margaret lived in order to know which church to check.
From army records
Richard's date of birth was Jul 12 1883
James' Dob was Jun 09 1890, and
Thomas DoB was Apr 01 1892
Birth certificates are only for the person who is named on it and their parents, unless it is over 100 years.
These DoBs are all over 100 years and their birth certificates should give:
Addresses
Mother's maiden name
They would also prove or disprove if Richard was a full or half brother
This would also help place them to pick a parish in which to start a search for the marriage