So, your father is William (1919), your grandfather is William (1888) and your great-grandfather is John (c1858-c1899/1900).Your grandfather might have been alittle young for military service in 1899-1902, but your great-grandfather would have been of age at that time. Perhaps your grandfather served during World War 1?
I should be able to find a marriage for William Devlin and Miss Lewthwaite, but I can not. Was she married previously, and therefore had her married name on the record?
I do not know if my Devlin ancestors, on my mother's side, are related to your Devlins, or not, but my Devlins came from Omagh, County Tyrone.