For three sows you need a 6ft hutch. My advice is actually to not get a hutch with a run which is permanently attached ie one where the hutch is above the run and connected via a ramp (hutch/run combo) but to instead get two separate items.
This is for several reasons - moving a hutch/run combo is pretty hard work.
They also cannot have access to the run all the time so for you to be getting access to the hutch in winter (when they can’t go in the run) via climbing through the run gets rather tedious!
Not all piggies will use ramps
Home and Roost (online) sell a 6ft chartwell hutch with compatible run which adjoins the front of the hutch when needed. You can then just disconnect it (they are connected via hooks) when necessary.
6ft Single Guinea Pig Hutch Chartwell | Fast Shipping | H&R
We would always advise that you rescue piggies rather than purchase where at all possible, so if rescuing is something you can do, then I would suggest you contact a rescue centre before purchasing any accommodation. If you buy something which doesn’t accord with a rescue centre requirements then of course they won’t rehome to you.
Make sure you buy corresponding thermal and waterproof hutch covers as piggies need a lot of protection particularly if they cannot be moved into a shed for winter.
Finally, if you do have a lot of foxes, is there any way you can keep them inside the house or indeed inside a shed or unused garage? Preventing an issue by keeping the piggies safely locked away is obviously better!