Okay....your dh may be able to preboard since he uses a cane. So I would do the online checkin, right at the 24 hr window. You will most likely get numbers in the A boarding group. Then, when you get to the gate, check in with the gate attendents and see if your dh is allowed to preboard due to his need for the cane. If he is, they may allow all three of you to board together....there's a very good chance that would happen.
If not, then just get into your assigned spots. In all probability you will be either next to each other, or very close to each other. If, for instance, you get spots A21, A27, and A30, just go back to the A32 spot and tell everyone to move up a spot. Then you'll be boarding in the A30, 31, and 32 spots. Unless it's a family that would be split up, most people are happy to move up a spot or two.
But, it's more likely that you'll be able to preboard. So, just ask.
As an aside....even if you all got various spots within the A group, and didn't ask others to move up, you would still be able to get seats together. That first person would just board, and choose the seats they wanted. They could save the addlt two seats in that row for the two other family members.
It's really only those in the C boarding group that have any issues finding seats together.