The house (A) is three times bigger than that one (B).
The house (A) is three times as big as that one (B).
The house (A) is three times the size of that one (B).
Do the three sentnences all mean the same, that is, A = 3 B?
I suspect tahy all mean the same, but the first one could also be interpreted to mean that A-B = 3B, so A = 4B.
They all mean the same thing. Guest is right that 4B also makes sense for the first sentence, but it's not used that way. We hear "three times" and we think in our heads "3*x", so whether or not it really makes sense, that's what the phrase means.
I would prefer the second or third sentence because it removes the ambiguity altogether, but, in practice, it doesn't really matter much... I never even thought about it before, really. If you were writing a technical paper or something where it's important to be precise, then it would be best to avoid the ambiguity, but otherwise, it's not a big deal. :)