There's an app for that! I have an app called walk logger that works like a pedometer, counts your steps and calculates how many miles you've walked, how many calories burned, great app!
How many times around 3 acres (walking the perimeter) until I've walked a mile?
Depending on how the acreage is formed it usually takes about seven laps per acre to make a mile. So with three acres it would be approximately 2.3 laps to make a mile.
Both Jewels Vern and Rooster Cogburn have pointed out the tricky part about answering your question.
An acre measures a precise area, but the perimeter of an acre depends upon its shape.
Either use a pedometer or app for a very good approximation, or drive around the perimeter if possible. (Car or bicycle)