Let 1/T be the part of the whole work that each sibling does. We also have that the total time will be 1/T, so We have that 3/T = 1/3, so T = 9 hrs.
As they only worked for 115 minutes, and they were supposed to work for 3 hrs, they only got 115/180 = 23/36 of the job done.
Two people working together will get the job done in 4.5 hrs. So, as 2 people working at the same speed worked for 45 minutes, we have that they got 45/270 = 1/6 more, or 6/36, which adds to 29/36.
Next, we have to find how many hours it takes one person to finish 7/36 of a field. The answer is 9 * 7/36, or 105 minutes, or 1 hr 45 minutes, so 3:40 + 1:45 = 5:25.
Correct me if I made any mistakes.