When you liberate areas, you will unlock challenges. Challenges are always visible on the map and you can also fast travel to them (if you're not in combat and have fast travel available). Many challenges will become easier once you obtain new MODs, vehicles etc. but you should be able to get 3 Gears quite easily in the early challenges without upgrades. 3 Gears is similar to getting 3 stars out of 5.
One of the easier challenge types is Crash Bomb, which is marked with a bomb symbol on the map. The objective is to crash your super-charged vehicle into the marked objectives and jump
out of the vehicle just before the impact. Just find the best route from start to finish and you should be able to get 5 gears fairly easily. Except in Laguna Blast, that one blows.