how do I switch between two email accounts on my iPad?

You don't...at least not in the way I think you're thinking.

For example, I have 3 accounts in my mail app. I've added all three via the settings 'mail, contacts and calendars'.

I then tap on the one I want to read. ANd I think I could add more accounts if I wanted to.

I'm gonna guess you want your password protected e-mail and her to have her password protected e-mail, and that's not possible if you use the mail app. The iPad was designed to be a single user portable iTunes portal and there's no accommodation for separate accounts.

One way you can have both accounts on there but have them password protected would be to use any webmail interface your provider has, or to use mobile safari to access your mail and have it so that you always need to sign in.

If you mean that you only want to view your own emails, and then have your wife view only hers, you can't. The iPad is not a multi-user device, so there's no way to set it up to keep user data separated. You can set up the iPad's mail settings to both accounts, but all emails will be viewable to both of you.

The only way you could perhaps keep them separate would be to use the built-in mail app for one of you and then get a third-party email app for the other person. That won't keep you from being able to see each other's mail, but it would at least help keep it all separated.

Regards.