Replacing individual cogs: I've been doing this for decades, with no apparent deterioration in shift quality. If you mix Hyperglide with the older Uniglide cogs, you will notice some clunkiness in the shift speed.

As to Campagnolo: check the letter coding on the 3 cogs you currently have versus what you want to order. I always wear out the 17-19-21 'A' labelled cogs on the 13-29 10-speed cassette. Like for like. Now cassette lifespan is doubled.