There is not enough information to calculate this.
Knowing the weight ratio of the fox to coyote as 3:8 in no way allows you to know the respective ratio of the wolf. To know the weight of the wolf would require knowing its ratio value, then the weights of all 3 is an easy calculation.
Example - 3:8:15 (f:c:w) is a plausible ratio based upon real-world weight averages for certain species/subspecies of the three.
- knowing the values of the 3 terms as 3:8:15 gives a total of 3+8+15 = 26 ratio values
- you then simply divide the total weight by this ratio value total; 120/26 = 4.62
- so each ratio value is 4.62 units of weight***
- now, simply calculate the weight of each canid by multiplying its ratio value by the unit of weight...
fox; 3 x 4.62 = 13.86
coyote; 8 x 4.62 = 36.96
wolf; 15 x 4.62 = 69.3
Validate the ratios by adding the weights together (we should get 120) 13.86 + 36.96 + 69.3 = 120.12
The total is slightly out because that 4.62 figure was a rounding up.
Now, the thing is, there is nothing given that allows us to know exactly what ratio value the wolf should be, I chose 15 myself because that is a real-world plausible value when compared to 3:8 for the other 2. Changing it to 16, say, means that there are now 27 ratio values total giving a ratio value of 120/27 = 4.44 obviously changing the weights of all 3.