Which adjective word can I use to describe my friend's father's military job in which he need to frequently resettle in a new place after every 2-3 years?
There's the word transient, but I wouldn't use that to describe a military job. (I think transient implies something less structured than a military position – when I hear 'transient worker', I think of something more along the lines of migrant agricultural work, and not something associated with a larger entity like a federal government.)
I'm not sure there's a single adjective that accurately expresses the sentiment you're after. Words such as unsteady are even more misleading than transient. I'd be inclined to say:
It's a job that requires frequent moves.
and leave it at that.
I know. None of the below are adjectives.
The only appropriate adjective, transient, has already been suggested by J.R
I know. I have not answered the OP question.
However, these are all expressions which can be used to talk about any person or any family that often moves to a different town or location. I think they are useful to know.
to be frequently relocated
to be (constantly) on the move
to be often transferred
to have no fixed abode
to be temporarily based
to flit from one town to another
to have temporary lodgings
You could consider the adjective itinerant, meaning "traveling from place to place." However, to my ear it has a slight connotation of unreliability, as if the person wasn't successful in any one place for long and was forced to move on. Requires frequent moves is probably safest, and easiest to understand.