My tuppence-worth:
I have just done this on my series III. One key for all three door locks, with the new kit coming from John Craddock.
On a 1982 series III, the ignition key is completely different pattern, length and width to the door locks so you will need a different key for this.
The only thing to watch out for is whether you have the standard / early model slam door locks or the optional / later model anti-burst optional locks. If you have anti-burst locks the part number for the barrels is 395142 for a 2 barrel set or 395056 for a 3 barrel set. If you have the earlier standard locks I have a funny feeling you need to replace the whole lock and handle mechanism which was 337801 and 337802 for left and right hand and you may end up with a different key for the rear door. (This is how my land rover was when I bought it before upgrading to anti-burst locks on all 3 doors).
If you have the old style locks and probably need to replace the whole thing anyway, (unless someone here knows better) I would really suggest that you take the opportunity to upgrade to anti burst locks. They are a direct bolt-on fit. If this is the case let me know and I will post a link to the Optional Equipment catalogue so you can work out what other bits you will need.
Cheers
Alistair

