ARe you saying that only the tach bulgs are out - or all all the cluster lights out. If all, check the rheostat first. If only the tach lights you can check the cluster's connection(s) first but you probably going to need to take a peek inside.
It might be frreaky co-oincidence that the three tach bulbs went out. You you can try replacing the bulbs. You will need to separate the two circuit boards when you open the cluster. There are three ribbon cables you have to disconnect so you can check them and their sockets as well as look for a crack in the board or cold/bad solder joints. If you search the forums there are a number of posts identifying a few solder points that often fail, so you might want to touch them up with a solder iron while you are in there.
I did a cluster LED post in the Mods forum that has a few pics of the disassembled cluster if you want to familiarize yourself with it before pulling it apart. I think I identified power and ground leads so you can bench test the bulbs.