The time it takes for you to hike 3 miles depends on your hiking speed but if you hike at 2 miles per hour then you would complete the 3 mile hike in 1 hour and 30 minutes.
But if you speed up your walking pace or hiking pace to 3 miles per hour which is the speed that most people hike at you would complete the 3 mile hike in 1 hour.
But of course some people stop to take a break and may rest around 5 to 10 minutes a few times during the 3 mile hike so it might take you 1 hour and 20 minutes to hike the 3 miles at 2 miles per hour hiking pace.
And maybe 1 hour 30 minutes to hike 3 miles depending on how long your breaks are.
But without taking breaks during the hike you should complete a 3 mile hike in either 1 hour and 30 minutes to 1 hour depending on your hiking pace.