You can't walk 3 miles in under 30min with a 45# ruck. I used to get guys ready for SFAS. A really fit guy who trains, on a gravel road can get slightly under 12min miles, without running. Any faster than that and you've got to break into a slow trot.
Army SF standards used to be every ODA member could maintain a 14min/mile pace with a 45# ruck plus web gear and rifle for 12 miles.
During smokejumper training in the 1960's, we had to carry our packs (110#) 3.5 miles back to camp in 90 minutes, and this included fording a stream and packing through underbrush. I came in 3rd from last, out of 21.