I have a set of the Zipp’s with a 404 front and disc rear. This is my third year on them and only use the wheels for races. There are probably 400-500 miles on the tubular tires (Zipp) and I am noticing some significant wear on them. How many miles can you expect out a set of tubulars? Seems like I get a whle lot more out of my training clinchers.
It depends on road conditions and how much you brake. If race results are important to you and the tires are worn, new tires seem like a cheap investment in terms of the time and money we put into the sport for training for a specific event. I use my clincher training tires until the tread wears out. For racing tires I would replace much sooner because problems during a race that I could avoid with a little pre race bike inspection are problems I want to avoid. For tubulars you should inspect a week before the race to give yourself enough time to get new tubulars tires if necessary and glue them on appropriately and not feel the need to glue tires on the night before an event.
Also if you glued those tires on three years ago it is possible the glue on them is no longer any good for keeping the tire on the rim. Some folks recommend using glue for no more than one year. If I change out my race tires early I rotate them into the training wheels or as spares.