This is a rubric (instruction) for when to sing a hymn. I translated it as "At Sext on third and all doubles." This doesn't seem right to me because the 'third' is redundant. I would like to know what the literal translation is. Thank you.
It would be nice if you could give us a little more context. Where did you get this phrase? What book is it from? When was it published?
It literally means "At Sext on Thirds and all Doubles". I know what Doubles are, they are a kind of feast with a high liturgical rank. I do not know what Thirds might be.
Update: Thanks for the context supplied in your comment below. It confirms in my mind what Brianpck said. There is apparently something called a "Third Double" as well as something called a "Complete Double". I've never heard these terms before, and I cannot tell you what they mean.