I have heard the Twa Corbies is older than the Three Ravens. However, my source for that, impeccable as it is, dates back to the '60s so it is likely more recent research has discovered otherwise.

Thomas Ravenscroft published Melismata in 1611 but the imagery in the Three Ravens suggests, to me, it is much older. The measuring of time by reference to the monastic offices would suggest a pre-reformation origin for a start.