Art of War 3 has many unit / buildings stats whose complexity goes far beyond a simple damage and HP system, and while some are not affected by positioning and formations, others are. Most notable for this are range and splash damage as they can make a huge difference in battles with large numbers of units.
Splash damage refers to the ability to damage multiple units in a small area at the same time. This can be an especially big threat if infantry is used, as they move twice as close together compared to vehicles, therefore also making them also twice as vulnerable to splash damage.
For vehicles it is recommended to make sure that they close in on the enemy position in a straight line instead of keeping them bunched up at one point, as this ensures that every unit only has at most 2 others units directly adjacent to its cell, that could be damaged by splash damage.
Another option to do this is to split the army apart into multiple smaller groups and to try and surround the enemy army with them, something that works especially well with infantry as it is both very mobile, and in case of the Resistance it even has the ability to move inside forests and spread out even more. This causes the splash damage to be split among multiple groups instead of just one target group and also helps defends against it.
Where it also matters, even though not immediately obvious, is that for jets, besides Bombers, are the only unit type that is not restricted to movement within cells. This means not just theoretically, but in a lot of cases also practically, it can easily happen for a big number of jets to bunch up so much, that any splash damage dealt by anti-air or other jets will hit all jets at the same time, which in the worst case can even lead to thousands of resources being lost in a single instance. To avoid this it is recommended to always let a single jet fly in front of the other jets as most of the anti-air will then focus on it, therefore being unable to splash damage the rest of the jets.
While the rules for avoiding splash damage also apply for tier 1 naval units, one doesn't have to worry about it with Tier 2 and higher naval units, as all of them have a bigger than usual hit-box, that already enforces a 1 cell gap between each ship.
Range advantages can generally be divided into engagement forcing and damage avoiding based on how they are used.
The idea behind forcing an engagement is to have a few high(er) ranged units like Torrent or Mammoth to start damaging the enemy's army from afar until they feel forced into attacking your siege units. If there is then an army in a good formation, the enemy will be forced into either retreating to stop the constant damage or to engage the army, which will usually lead to a bad engagement.
Avoiding damage on the other hand is usually dependent on having superior maneuverability and can make use of shooting mechanics like being able to shoot while moving. For example it is possible to keep Heavy Assaults and Grenadiers exactly so close to Armadillos and Hammers that while technically in range to shoot, their aiming time will usually be too long for them to actually fire, which in turn will allow the vehicles to slowly wear down units that are usually their counter.
It's a similar case when Solaris is used, as it is both the highest ranged (except siege units) and also second fastest (Coyote) ground unit, therefore allowing it to always stay just barely outside the enemy unit's weapon range, while also not allowing them to approach close enough to fight back due to its superior speed.
Another not so obvious way to avoid damage with range is unreachable terrain, an advantage several units can make use of.
For example Cyclones out-range Armadillos and are able to move over cliffs and on top of inaccessible hills. So luring units to the edge of that terrain barrier can allow it to shoot down the enemy without them being able to fire back.
It is the same for Deltas which can retreat and approach through a beach, therefore allowing it to keep distance even from units that are faster than it like for example Riflemen.
Something similar is possible with Snipers and Fire Assaults on some maps like The Lost Road, as some forests can be so big that the Sniper can be hidden so far back, that the Fire Assaults can not reach them at all, no matter where they move to.