If you do all the side quests you will eventually get the Medal of Exploration I, II, and III regardless of what I post down here. The planets involved for these trophies are generally the ones you drive the Mako on that don't have a separate trophy involved. I don't know if there are any side quest planets that don't count but I discovered by accident a way to cheese these trophies and get them faster for those who want to. I only landed on 5 planets and got all three trophies. If you land and then go back to the Normandy and land again on the planet it counts toward all three trophies.