Your looking at it! and yes, its only 3 ingredients! mixed nuts (I like to buy two cans of nuts at a time at Sams Club a lightly salted peanuts and a can of mixed nuts.) Raisins, and your choice of M&M's my mix isn't a science but it can seem like it. I first mix the two cans of nuts together, then keep one can of the mix mixed nuts for later. Then add the raisins to the mix mixed nuts, get the raisins coated in the salty nut powder, then add the M&M's I use about the same ratio to raisins to M&M's. Sometimes i like to use peanut butter, or peanut or plain M&M's. I use this all the time for on the go snacks with those little glad or ziplock containers that hold 1/2 cup. I reuse them often for this purpose.
Another alternative is to use crasins instead of raisins, I do this if Crasins are cheaper than raisins and instead of M&M's you could use Reses Pieces, this is really good around the fall season.