Quite often they will cast a small fracture in a foot for pain relief as much as for healing. I had a small avulsion fracture off my 5th metatarsal a few years back. Walked - or should I say hobbled - on it for about 6 weeks before I finally sort advice from one of the surgeons at work. He had a cast put on it Friday afternoon but the plaster tech botched it and I had a crease on the inside which was mega painful! That baby was off by Saturday noon and didn't go back on! It healed just fine.

As for the OS talar and navicular - OS is just a term for bone. It's not uncommon for doctors to prefix the names of small bones of the foot with it. As in Os Calcis or calcaneum, the heel bone.

Also, I would have thought that 3 weeks was being optimistic. 6 would be nearer the mark, especially if you're still walking on it. Lateral malleolus fractures also mean the ankle joint is potentially unstable. If it's not been internally fixed, that will need time to heal as well.