When I buy, I ask to be sure. In many cases, quite a few hens are in with the same cockerel so if you get a trio, chances are they're half siblings rather than full siblings. WIth ducks there can be two drakes in together as well so then the odds are even better. In some breeds, poultry is intentionally inbred to bring out certain features and often if it's very rare breed, it's hard to avoid anyway. There was an article a few months back in one of the smallholding or poultry magazines about the size of the gene pool for various poultry breeds and you could clearly see how interbred some were.
Anyhow, I'd say it wasn't ideal but it isn't uncommon. Most breeders just couldn't keep three different pens of each variant of the breeds they do in order to supply three completely unrelated birds (financially it wouldn't make sense and they'd no doubt end up with the wrong balance of numbers anyway). But you don't want to be repeating it generation after generation. So the best thing is to bring in your own diversity as Shygirl suggests. I'm hoping to breed West of England geese which are really hard to track down. I've got one pair that are related via one grandparent which is the best that breeder could do and I'm going to get a second, unrelated (to the first pair, and only distantly to each other) pair later this season. Then I can sell unrelated pairs myself but it's still tricky to keep juggling all the potential outcomes from their offspring - particularly as I'd like a trio of each ideally and it would be easiest if I could keep two of my own females and just swap them over. We'll see how that one goes....or if they ever manage to produce any fertile eggs which they didn't manage this year.
H