3light-years =

1.8 x 1013 miles

1.8 x 1014 miles

1.8 x 1015 miles

               ( 3 yr) · (186,282.397 mile/s) · (86,400 s/day) · (365 day/yr)

           =  (3 · 186,282.397 · 86,400 · 365)  mile

           =      1.762380502 x 10¹³  miles

           =      1.8 x 10¹³  miles    (rounded to the nearest trillion miles)

The duration of a photographic flash is related to an rc time constantly