There are three significant figures in 0.00100.
Below are links to some online counters that you can use in determining the number of significant digits:
www.chem4free.info
ostermiller.org
www.sigfigscalculator.com
There are three rules on determining how many significant figures are in a number:
Non-zero digits are always significant.
Any zeros between two significant digits are significant.
A final zero or trailing zeros in the decimal portion ONLY are significant.
While it is important to know how accurate a measurement is, it is also important to convey one's degree of confidence in the accuracy of a number when making an observation or carrying out a calculation. This can be accomplished by noting numerical observations or quoting results to the correct number of significant figures.
See ChemTeam.info's Significant Figures Menu and Dartmouth.edu's Use of Appropriate Significant Figures to learn on how to work on these digits.