It says that you can roll up to three extra dice to dispel for spells cast at the unit/model with MR. So for MR3 is that actually three extra dice for every spell cast at it? Because that seems pretty unbalanced. Or is it three extra dispel dice per enemy magic phase that can only be used on the unit with MR? Or is it like the last option but over the whole game?
The 7th Edition Rulebook specifically says, on page 95: "The number in the brackets indicates the maximum number of extra dice that may be rolled when trying to dispel each spell that affects the magically resistant model." I imagine this is balanced out by the fact that relatively few units have the Magic Resistance special rule, so you're prompted to use your magic against other units and think of other ways to defeat units with Magic Resistance, i.e. mundane shooting or close combat. And welcome to Lustria! Nice to see a 7th player join the forum! I started in 7th and still have a fondness for it, mainly because the armies I play (Dwarfs, High Elves, Lizardmen, Skaven, Greenskins and Beastmen) avoided being given the cheese books.