This has only only three sig figs because it is 546.000 the zeros are just place holders so thus meaning they are not significant. It is just another way to write 546 because in that number there is also 3 sig figs. They are just trying to throw you off. Just watch out because if the example is 540 then it is just two significant digits, and it were to be 540. then it be three cause that "." makes the zero significant.