Chapter 1 - Sections 1.1-1.8 - Exercises - Problems by Topic - Page 40: 78b

There are three significant figures in 312,000.

All non-zero numbers are significant (in this case the 3, the 1, and the 2). The three zeroes are insignificant because you could just as easily write 312,000 as $3.12 \times 10^{5}$.