It sounds as if you've been reusing old compost from various pots, is that right? Given that you've got problems on all three of your herbs, all in separate pots, and presumably all planted in old compost, I would indeed suggest you bin the lot, including the compost, thoroughly scrub and disinfect the pots, refill with new potting compost and new plants. I don't recognise the problem on any of the plants, but since its affecting all, its most likely cultural.
Otherwise, I'm assuming the pots have good drainage, that you water thoroughly when the surface of the compost is dry to the touch, but don't leave them standing in outer trays containing water for longer than 30 minutes, and that they are in full sun. Note also that Rosemary needs repotting every year, and that mint will start to suffer in a pot within 2 years, and you may find you don't have many leaves on it because the centre portion has died back. When that happens, turn it out, replant the outer stolons or portions in fresh compost.

wow. what a great and full answer! I'll try to get to it at the weekend. i didnt know that about the rosemary. I thought that it grows into a massive bush.

Rosemary does grow into a large bush, 6 x 6 feet usually. Are you confusing what I said about repotting rosemary with the rest of the penultimate sentence, which refers to mint?