Others may disagree with me, but the best of those three would be the PC-7B, which I have. The big advantage that this case has over the others is front and rear 120mm fans, for excellent cooling and great potential for a silent computer. The PC-65B has a front 120mm fan, but the rear and top fans are only 80mm, and the PC-61 USB has only 80mm fans throughout, with two in front, one in the rear, and one up top. 80mm fans can't move as much air as a 120mm fan (or at least, they can't do it as quietly) so if you want good cooling without a buttload of noise, the PC-7B is your best bet.
The reason the PC7B is cheaper than the other two is because it doesn't have a removable motherboard tray, which the other two do. There's plenty of room in the case so I don't think it's really necessary, but if it's a dealbreaker for you, you should look elsewhere.