If you have 3 “7”’s and destroy them all, you get 2100 Life Points.
If your opponent also has “7”’s, they are also destroyed.
If you have 3 “7”’s and activate their effect, your opponent can chain “Imperial Order” to negate the effect.
Each effect of “7” that gives you 700 Life Points goes on a chain, so if multiple “7”’s are sent to the Graveyard simultaneously, each effect is 1 step in the chain.
If other cards are also sent to the Graveyard at the same time, like “Sangan” or “Black Pendant”, the card controlled by the turn player is Step 1.
If your 3rd “7” is activated, but destroyed by “Mystical Space Typhoon” in a chain, you can’t activate its effect, but you still gain 700 Life Points for its Graveyard effect.