"3-Hump Lacooda"'s effect is an Ignition Effect.
You don't have to activate "3-Hump Lacooda"'s effect just because you have 3 copies in play.
You can choose which of the "3-Hump Lacoodas"' effects you are activating; it can be one of the ones you are Tributing or the one that is remaining on the field.
Tributing 2 "3-Hump Lacoodas" is a cost.
If your opponent chains "Ring of Destruction" or "Book of Moon" to the activation of "3-Hump Lacooda"'s effect, the effect is not negated.
If you Summon your third "3-Hump Lacooda", you can activate its effect before your opponent activates "Torrential Tribute".
7
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.