lambs aren't always born as multiples but most are. I think as I recall from my animal science classes, a sheep is more likely to have 2-3 offspring than it is to have a single lamb.
The thing is, multiple births is hereditary in sheep (and humans for that matter). So farmers would prefer an ewe that produces 3 offspring per lambing to one that produces a single. Think of it like this. If each ewe produced 1 offspring, the farmer could only have 30 offspring a year total. (actually he probably could have more because sheep often are bred more than once a year). However, if he had ewes that were known for multiple births, he could reduce his flock (and thus, his overall costs as well) to 10-15 ewes. Also he'd want to keep the female lambs that were from lambings that had triplets or twins, and perhaps sell the ones that were from singles, just because genetically, the multiple birth lambs are most likely to be multiple-birth producing ewes when they are at breeding age.
Never trust a tall dwarf... he's lying about something.