Well, the starting point is not to assume that you have to worm at all. We should only use wormers if there's actually a problem with them. All sheep carry worms, and an adult sheep has usually built up a sufficient immunity for it not to be a problem for them at all, so your two older sheep are probably fine anyway. The lamb may benefit from worming, but not necessarily. If the lamb remains in good condition, and is not scouring, then the probability is that your stocking density is so low, and the pasture sufficiently clean, that you wont need to use wormers.
DEFINITELY don't use them 'just because'.
If you think you may have a problem take a sample of poo to the vets and ask for a FEC (fecal egg count) to be undertaken (they generally do it themselves, on the premises, so you should get a result within 24 hours - and the vet can advise you whether you actually need to use a wormer, and what would be the best brand/type for your sheep and the local area). A test is likely to cost around £15.