How to complete scout 3 different cities in the world?

Make sure you have built the sentry tower building and then you can tap on any base in the map (not in your alliance) and choose the scout option. This will send a scout legion to the base to check on its stats. (number of troops, resources etc). It does not attack them but it will warn them that they have been scouted so they may think its an attack. Probably safer to pick lower level bases. I think for the quest though you need to make sure they are level 6 or above.

The info you get back from scouting will depend on the level of the sentry tower building. If you haven't upgraded it you wont get much back but for the quest you only need to do the 3 scouts.