There are only 2 ways of wrong labeling of all 3 boxes as shown in below table.
Line one- right, is the boxes with correct label, GOLD for box1, SILVER for box2 and MIX for box3. Line two and three describe the options of wrong labeling of all the boxes.
right GOLD SILVER MIX
wrong1 SILVER MIX GOLD
wrong2 MIX GOLD SILVER
By selecting any label you can guess the rest of the boxes correctly. For example, if you chose to open the box labeled Silver, you may get gold or mix in the box. If it is gold, go by wrong1 line for other boxes, it means the box labeled Mix will contain silver and the box labeled Gold will contain mix. If it is mix in the box, go by wrong2 line- the box labeled Gold will contain silver and the box labeled Mix will contain gold.
So you may chose any label and find the correct boxes easily.