Using variables to control many things (like health, ammo, etc) is a fundamental and basic part of construct 2, and programming in general.
Once you learn how to use these simple but powerful tools, it will all make sense.
I can try to make an example today (I expect a fairly quiet day in the office today) but I will have to see if time does in fact allow.
Essentially, you need to create a global variable on your event sheet (right click on the event sheet and add global variable) then name it HEALTH. Set the initial value to 3.
Anytime you want to subtract 1 from health, you have your condition for doing so... example: player collides with enemy ->(action= subtract 1 from global variable HEALTH).
Then you have another event that checks what the health is at: global variable health = 0 ->(action: destroy player)
You can use variables to track all kinds of stuff that you need to be, well, variable... so simple yet so powerful when used in certain ways.