Let f, s and g be the ages in years, of the father, son and grandson, respectively.
As I know that you're all going to get technical on me, I'll use 6 d.p. for my calculations.
In a year there are technically 365.242199 days.
This leaves us with 52.177457 weeks.
Months are fixed at 12.
(note: this also works if you use 365 and (365/7) for days and weeks, respectively.)
Age of the son in weeks is the same as the age of the grandson in days:
52.177457*s = 365.242199*g
Age of the grandson in months is the same as the age of the father:
12g = f
Sum of their ages is 100 years:
f + s + g = 100
Solving these, we get:
s = (365.242199/52.177457)*g
f = 12g
12g + 7g + g = 100
20g = 100
g = 5
s = 7g
s = 35
f = 12g
f = 60
The father is 60 years old, the son is 35 years old and the grandson is 5 years old.