If you are weighing your grist and milling the same day, chilling and pitching your yeast and clean up, there is no way 3 hours is conceivable for five gallons or more of all grain brewing
I get VERY close to 3 hours when I need to. My personal best was 3 hours and 25 min, completely finished other than putting away all my equipment (it has to dry after all).
Step by step:
1. Set up everything.
2. start heating strike water
3. weigh and crush grain
4. start mash
5. start cleaning
6. stir mash, then heat sparge water.
7. run off, sparge, runoff, take reading, start boil. ***
8. weigh hops
9. clean mash tun, dump grains
When boil is complete:
1. set up chilling (two buckets of tap water, third bucket is ice water running through immersion chiller with pump)
2. clean everything else.
3. pour into fermentor, take reading, pitch.
4. done
*** I am soon going to start using a second pot for my sparge water so that I can run-off right into my boiling pot and begin my boil while I'm sparging. I think this will cut about 10 more minutes off when I need to save time.
Trust me, it's doable, but any less than 3 hours and you have to cut mash/boil times, which I won't do. I rarely try to go this fast, but sometimes I brew after work and I have to so I can be done by the time my daughter is going to bed. I brew alone btw, which I've learned actually SAVES time.
EDIT: wanted to add that my friend modified my Bayou burner so that the pot sits only about 1.5 inches off the flames. This not only greatly cut down my time to temperature, but also extended my propane tanks to about 6 batches per.