Yeah, despite A!MG-Urd being almost, but not quite,†entirely unlike a crone, she should definitely be the eldest. The feel I'm getting now makes Bell the eldest.
†: I feel I should explain: in Ah! Megami-Sama!, Urd has all the non-physical aspects of the "Other One", both mystical (culling, preserving, things associated with autumn and winter) and mundane (off-colour humour, meddling in others' love lives, general lechery). Only her physical appearance – arguably the least important part of being a Crone – fails to match.
†: I feel I should explain: in Ah! Megami-Sama!, Urd has all the non-physical aspects of the "Other One", both mystical (culling, preserving, things associated with autumn and winter) and mundane (off-colour humour, meddling in others' love lives, general lechery). Only her physical appearance – arguably the least important part of being a Crone – fails to match.