Bill's right: the tank still contains 3/4 of its fuel. You might be confused about how that relates to the first half of the sentence. If you poke a hole in the bottom of a gallon of water, it will leak quickly when the gallon is full, and it will leak more slowly when the gallon is almost empty, because there is less water pressure. So, if the tank is leaking quickly, it must still contain a lot of gas.

It's probably got three-quarters of a tank left (remaining). I understand from the English used that 3/4 is remaining in the tank because the word got means the tank still possesses fuel. If it was out of the tank the word lost or something similar would have been used instead of 'got.'

It's not really a normal sentence so don't worry about it; it should be 'there's 3/4 of a tank left' or something in a normal situation. Also, what is 'it'? Maybe from the context we could understand, for example if a big alien is sucking the fuel out, 'it's got 3/4 of a tank' means the alien has taken 3/4 of the tank, you know?..