To solve this I would use scientific notation, this way you don’t loose any information. I mean that’s why it was invented in the first place.
-3.0e-3 has two significant digits
-3.0 .10^-3 has two significant digits
In general leading zero's do not count as significant digits (but they do take up space). Trailing zero's however convey information on up to how many decimal place the measure is accurate, so you should especially watch out with those.