One thing I liked a lot about ME2 was that I didn't have to hunt across a whole planet to find side missions. When you began a scan the landing site would pop right up. I'm hoping it is, but I figured I'd ask, is it the same, or do you have to scan around each planet? Or is it completely different?
Scanning a planet now gives war assets and other rewards.
I do not believe there are any side-missions obtained from planet scanning in ME3.
Most side missions are obtained from a small set of methods
While the main line quests are labelled Priority: the side quests are labelled in a variety of manners, which often suggests their origin; Citadel: missions usually are obtained on or completed at the Citadel, N7: missions often appear in your journal as a response to other missions or Priority milestones, Name: missions are usually scanning or short combat missions if the name is a sector or planet or a sequence of interactions and sub missions if it's a person's name (eg Aria's quests).
A reasonable list of missions can be found at the ME Wiki.