Williams is back in Broken Valley, if that helps. I've heard it's possible to make a bad choice with Laura when you first see her, making her disappear forever, but I can't confirm that. Normally, you first see her on the beach arguing with a friend about a ring, after that she is up on one of the ledges. You need to scour those cliffsides, there are lots of little caves and hidden areas. Arthur is in one of them, and the other two are in or near one of the many wyvern nests. But of all those, WIlliams is the only one you need to finish the game, the others are optional.
There is a known bug that causes triggered NPCs to appear in the wrong spot, and usually that spot is nowhere you can reach them. Tech support can fix those issues if you e-mail them your save game, and they are very quick about it. I had to do it twice, and both times I sent it in the evening and had it back the next morning.
