Where the three comes from
It is a count of what is published here, nothing more. Three strings were supplied and three strings are printed. This site did not discover them, did not test them and did not choose between them.
Because of that, three is a number about this page rather than about the market. The market may run more addresses than three. It may run fewer that still work. A count of published addresses and a count of live addresses are different quantities and only the first one is knowable from here.
What three addresses buy you
Redundancy against the ordinary failure, which is one address becoming unreachable while the service behind it is fine. Onion services can be under load, can be under a denial of service attempt, or can simply have a bad introduction point at a given moment. A second string to try costs nothing.
They also buy you a comparison. If you find a fourth address somewhere and it looks nothing like these three, that is information. Not proof, information. The market chose recognisable prefixes for a reason, and a candidate that shares none of the pattern deserves more suspicion than one that does.
What they do not buy is confidence. Three addresses that all lead to the same impostor would look exactly like three addresses that all lead to the market.
What people get wrong about it
The most common error is to treat the first one in a list as the main one and the others as spares. There is no such hierarchy here, which is why nothing on this site is numbered or labelled. Every one of the three is a full address for the same service, and none is a fallback for the others.
The second error is to assume a longer list is a better list. Sites that publish twenty addresses for one market are usually publishing every string they have ever seen, including old ones and hostile ones. A short list that is honest about where it came from is worth more than a long list of unknown origin.
The third is to treat the count as a health signal. Three published addresses does not mean three working addresses, and this site will never claim otherwise. It runs no checks at all.
What would move it
The market retiring an address, adding one, or rotating the set. None of that is announced to this site, so the count here changes when the published list changes and not before. If you are reading this a long time after it was written, treat the three as what somebody had on a particular day.
The date at the top of the underlying data is the honest marker. There is no checking date, because no check happens.
Questions people ask
Which of the three should I use?
Whichever one responds. There is no preferred one and this site does not know which of them is answering at any moment.
Why not publish more addresses?
Because more addresses from unknown sources would make the list worse, not better. Every extra string is another thing a reader has to trust.
Do the three lead to the same place?
They are published as addresses for the same market. This site has not verified that and cannot, since it does not connect to any of them.