Unless the Tornado is for a strange new reason carrying super-size 'practice' bombs, 3kg is indeed about right.
They have the same ballistic characteristics as the full size item, and being able to carry 4 on each of possibly several carriers gives the aircrew a few goes for training.
Hollow steel body, on test ranges a frangible nosecap with a 'smoke & flash' tiny firework is fitted ( about the same effect, really, as a bag of flour ) to give a visual cue for cameras & ground observers as to where it hit re. the target.
No explosive potential whatsoever, but it would disimprove one's day if supremely unlucky enough to receive one on the head.
On ranges, any accidental release, usually by incorrect switching but sometimes possibly by a fault in a stores management system, required a full brief from the Aircrew & Flight Test to show the Range Control ' what went wrong & why it isn't going to happen again ' before they were allowed to continue trials.
No particular blot on a pilot's ( we were Harrier trials team so no navigator -W/O ) record - it was a high stress environment with new aircraft or systems or both - but if he did it twice questions might be asked...
As to losing a practice bomb on a cross country flight, I imagine most of the above still applies.