Answer: it will take a combined rate of 2.67 hours
Explanation:
Father can do the job on the farm in 6 hours. This means that the father's unit rate of working will be 1/6
The older son can do it in 8 hours. This means that the older son's rate of working will be 1/8
The younger son can do same job in 12 hours. This means that the younger son's working rate is 1/12
When they work together, they will be working simultaneously. It means that their unit rates are additive. The combined unit rates will be
1/6 + 1/8 + 1/12 = (20 + 15 + 10)/120
45/120
Assuming it takes t hours for them to complete the work while doing it together. Their combined unit rate will be 1/t
45/120 = 1/t
45t = 120
t = 120/45 = 2.67 hours