Hi guys !
To get 3 stars on a level, you have to:
- kill all the enemies in the level
- rescue all the hostages
- not die.
The 1st level is (obviously) the easiest one.You have only 3 short sections with respectively 2,1 and 2 hostages to rescue.
And there's no boss,so perfect for the no death.
You can pause the game and look at the stats of each section you’re playing.It helps a lot to track the remaining hostages and enemies.
I also found this video for a 3 star run on the 1st level: (credits to: OniWalker)
Oh and this is not cumulative.You must do the 3 objectives in 1 run.
Have fun !