I though this would be a motoring question but it is regarding family I suppose. :).
A lass who works part time in our office has a festa. She doesn't use a 3 wheel system but manages to just get a 4 wheel buggy type thing in. She just has enough room for a nappy bag and a couple of smaller things. Definitely not enough room for a weekly shop though without losing the parcel shelf and stacking on top. Not healthy if you have a little one in a seat just in front.
If you have a new shape Fiesta, have you considered making a straight swap for something perhaps slightly older but more suited to moving a wee one around?
My twin bro and his wife had a mini that was ace to drive but completely unsuitable for getting a child in (3 doors and the front airbag doesn't deactivate). They traded and got a Merc A' class. The front AB can be switched off and there is room in the back for everything then.
Other models worth considering are the Vauzhall Zafira, CItroen Picasso or Renault Megane Scenic.
Hope this helps, good luck with the little one.