You cannot safely or realistically sleep 8 hours in 3 hours
Use a consistent sleep schedule every day
Go to bed and wake up at the same time
Keep your room dark, quiet, and cool
Avoid caffeine, nicotine, and alcohol before bed
Stop screen use 1 hour before sleep
Avoid heavy meals right before sleeping
Exercise regularly, but not right before bed
Use a short nap if you need extra rest
Prioritize getting enough sleep time instead of trying to compress it
If you cannot sleep enough, consult a doctor or sleep specialist