Answer is 26 bags
Step by step
First find the least common multiple (LCM) of the 3 nuts. First I checked to see if 26 was a multiple of 78 and 52, yes it is! 26 is the LCM.
26/26 = 1 almond per bag
52/26 = 2 peanuts per bag
78/26 = 3 pecans per bag.
So all 26 bags will contain 1 almond, 2 peanuts and 3 pecans.
Problem solved. Not a very big snack bag in my opinion.