The amount of water is 3l.
The amount of water has 3l.
This bottle of water is 750 ml.
This bottle of water has 750ml.
Thanks so much for your kind help!
It depends on what do you want to convey. Is it regarding the capacity of the bottle, or the actual quantity of water left in the bottle. The first two statements do not talk about the bottle at all and just the quantity of the water.
And obviously the second statement is grammatically wrong. In the sentence, has serves the purpose of possession and it doesn't go with the noun 'amount':
I would modify the statements as followed (excluding 2nd statement):