I played a Warmachine MK III demo game with a friend named Andy Mitchell last night. He ran a battle box version of Magnus the Traitor. I and Damiano and some warjacks.
The changes with Warmachine MK III are the following:
1) Point costs for figures have been doubled.
2) Warjack points for individual spell casters have been tripled.
3) 75 points is the average point cost for a regular game of Warmachine MK III. Battle box games as "zero" points. You use the Warjack Point allowance of your warcaster to come with your battle force.
4) You can now premeasure during your turn.
5) Warjacks gain one free focus point for being in the control area of your warcaster.
6) Some stats and abilities have been changed depending on a model's card.
7) Any Mercenary force can work together.
8) Everything else in the basic game is the same as MK II Warmachine.
Andy went first with Magnus. He moved his warjacks and then cast several spells. Not knowing what I was doing, I cast an upkeep spell and moved my warjacks. The second turn, he advance Magnus again and got another shot off on my heavy warjack. I had Damiano fire his personal heavy warjack's cannon back. I damaged one of his heavy warjacks. Magnus activated his other warjack and wrecked my heavy.
There are no more wrecked warjacks. You just take the model off the board. I had my remaining Bucaneer warjack and Damiano retreat behind hard cover for a turn before calling the game. There was no way I'd win...
Warmachine is a simple game to figure out once you know what the stats are. There are stats for Movement, Melee, Ranged Attack, Defense, Armor, Power/Strength, and Command. For Melee and Ranged Attacks, you subtract your MAT/RAT value from the enemy's model's Defense. That is the roll you need for successfully attack on 2D6. You can spend a warcaster's Focus to get another 1D6 on Attack and Damage rolls.
Damage rolls are handled similarly to Attack rolls. However, you take your model's STR&POW state and subtract it from the enemy model's Armor value. That is the number needed to damage it on 2D6. You also roll a different colored 1D6 for Warjacks to determine the location of damage on the model. So that regard, the game is like Battle Tech concerning the Warjacks/Colossals. Damage to living models and warcasters' are straight damage roll. Loss all your warjacks/or warcaster, then you lose the game.
Warcaster's also have focus points. That allows them to cast magic spells and boost their warjacks' Attack/Damage rolls. Focus points are allocated at the beginning of the warcaster's turn. You need to think ahead about how you're going to use your focus points. They regenerate each turn. Hordes has a similar thing called Fury for a warlock's Warbeasts, though they are used differently in Hordes as opposed to Focus in Warmachine. Both games are interchangeable...