Darren,
Just to elaborate on what Stew has said.
UNF/UNC (Unified fine & course) use a 60 degree angle for the threads, and is mainly found on American products, but can sometimes be found on other countries products. Britain used it as one of it's standards for a short time.
Whitworth/BSF(British standard fine) use a 55 degree angle, and are found on all sorts of older machinery and products, from all over the world.
Both of the above work on the TPI (threads per inch) principle.
Metric works on a 60 degree angle and is measured by pitch (distance from one crest to the next), and as you know is becoming the standard from everywhere.
So if they are cheap enough, get the lot, but you will definitely require metric. You can use either one of the others for rough gauging the missing one (they don't fit correctly because of the different angles), but you can compare the pitches easily enough.
Hope this has helped.
John