the 3d game that i will develop is the same as the game Battle City that often we play on a family computer. just need some help on making enemy tank AI to move randomly. Every answer will be very much appreciated. Thank you. 
One solution would be to use the NavMesh feature in Unity. You can then give your agent a randomly chosen destination with the SetDestination API.
If you want a really stupid AI :
you need some functions such as : GetRandomDirection, MoveForward and some variables such as : MinTimeBeforeChangingDirection, MaxTimeBeforeChangingDirection, Speed
If you want something more nice :
Check NavMesh, NavMeshAgent, and instead of getting a random direction, get a random Transform point within a list of positions that your agent could need to go.
(if your map is an ‘house’, or something complicated)
You can also mix these two solutions :
Get a random position then ask your agent to go there, using the navmesh.
Once he finished, do it again 
(if your map is a cube)
Happy coding