As we are using a joint bank account (and most of our utility accounts are in joint names), my wife has signed part 1 of the consent forms, I have signed part 2, and we have both signed part 3 - is that correct?
No, not quite.
The applicant signs Part 1, giving permission for checks to be done on them.
The UK sponsor signs Part 2, giving permission for checks to be done on them.
NO ONE signs Part 3... you leave it blank and do not include it with your documents.
Part 3 does not apply to your application... it is only for people who are providing Third Party financial support for the visa (i.e. friends or family). However, you are not allowed to use Third Party financial support for your visa because only the sponsor and/or applicant's finances can be considered, so there is no third party who can sign it.