Fumbles only come into play once it's been established that you have missed.
The order in which you apply the stages are as follows:
- Roll a number of dice equal to the ROF of the weapon and an additional Wild Die.
- Calculate how many hits you have. If the Wild Die is a hit then you can substitute that for a miss on one of the Shooting dice.
- If you have any misses then look to see if you rolled any 1's
- If you missed with any shots and rolled no 1s then there is no fumble
- If you missed with any shots and rolled at least a single 1 then you have a fumble
For example, lets say you are firing your ROF3 gun with 3d8 and a d6. The d8s come up 5,5,1 and the Wild Die comes up with a 7. Due to the -2 penalty for firing full auto, the 3d8s all miss. However, you can substitute the result of the hit result on the Wild Die for the 1, meaning that you hit with all three shots and there is no fumble.
Let's take a second example, same dice but the results are 5,5,1 and a 3 for the Wild Die. Again, you have 2 hits with the Shooting Die. However, this time the Wild Die is also a miss, so overall you have one miss. As you rolled a 1 and you missed, you fumble.
See here, here, here for confirmation of this.