Hi,
In a multiplayer game with bomb card, if multiple players put a bomb card on the same land, what does it do?

Does it reduces 75% if 2 bomb cards are used? 87,5% if 3 cards used? etc.

Nothing is played "Trully simultanously".
You can go through order by order inbetween turns.
If multiple bomb cards are played, they are still played one by one.
So if there is a stack of 100 units, and 3 bomb cards are used on it.
The stack is halved 3 times. So first to 50, then 25, and then 12 (or 13 idk if it will round up or down)

Thank you.

Would multiple bomb cards played by the same player does the same result?

player doesnt matter, as the function of the bomb card is always identical

Two bomb cards frequently does more than 75% damage because each card will individually round up.

e.g. if your opponent has 11 units, two bombs will reduce it all the way down to 2 units (11 -> 5 -> 2) rather than “just” the 3 units you might expect.