Speed of Tim = 60 miles/hour
Speed of Joe = 45 miles/ hour
Distance = Speed x Time
A 3 hours, Tim had travelled 60 x 3 miles = 180 miles.
At 3 hours, Joe had travelled 45 x 3 miles = 135.
The difference between the distances is 45.
Therefore, they were 45 miles apart after 3 hours.
I hope this is right.