Down here in CHCH we have a slightly different thing from MMH, it's called the Mothers and Babies service and boy is it amazing- they're looking at doing more in other parts of NZ *fingers crossed* that that happens!
Anyway I'm not sure how different it is from M&B but I know that it really helped me being under them- that practical help as well as free counseling (and to be honest if it wasn't free I don't know if we would have done it as DH had to leave his job when DD was 4 months old!) I do also know some people who felt it
didn't help, and so they left the service when they realised it just wasn't for them- and leaving was easy enough. Again, I know it might be different than down here, but it seems that they help people that want help- it's not forced on you, ya know? So I'd say give it a go- you never know what good it could do, and if it doesn't work out or you're not happy with what's happening then you can be discharged from the service and that's that!
With #3 I think all that would happen is that you might get some advice about how to prevent another episode of PND including what to do about meds if you are on them when #3 comes along, and they might give you more support if you do hit PND again as they would already have you on file.
Good luck with everything, I hope that your meds start to help you real soon and please keep posting here if it helps