Could just need locking nuts or some loctite on there but have you checked the engine mounts?

It's a common problem with Fiesta's with CVH engines which shift about too much on knackered mounts making the exhaust yaw open loosening off the nuts at the downpipe to centre section join. Might be something similar with your 3i - I had similar problems with my old Orion 1.6i Ghia.

HTH,

Andy