This is a very interesting question. According to the rules for determining significant digits it could be said that 0.00 has no significant digits as leading zeros are not significant, for example 0.0027 has only 2 significant digits.

But a measurement of 0.00 indicates the the measured quantity lies between 0.005 and -0.005

If a thermometer measures temperature as x.xx, there are 3 significant digits in the measurement. If the same temperature shows a temperature of 0.00 this too can be said to have 3 significant digits.

The number of significant digits in 0.00 is 3.