Distal diaphysis of 3 mm displacement of distal fragment. no evidence of solid callus ..... what does this mean of a broken foot?

It depends: That's not much information and would depend on the location and timing of the fracture. It's likely a metatarsal shaft fracture, just behind the toes. 3mm displacement means the pieces have a small gap between them and no solid callus indicates it isn't showing signs of healing...Yet. If this is new, not a big concern. It it happened a month or more ago, it might be a problem. Have it checked.

Depends on timing...: See a podiatrist for an evaluation and give specific dates for when the original injury occured and dates of treatments and x-rays. It may be a delayed union of the fracture.

Could be.: A displacement of bone usually indicates fracture. It could be a new or old fracture.

Fracture description: These means the end of the shaft (long part of the bone, probably a metatarsal) is fractured, and the end portion of the bone is slightly out of position. The lack of callus means it is not healed yet, as callus is new bone often seen at the site of a fracture.