Significant figures are all numbers greater than 1 so in the number 120, 1&2 are sig figs so it has 2 sig figs
All zeroes that are between 2 non zeroes are also sig figs so in 103, 1,0,&3 are all sig figs so it has 3 sig figs
Leading zeroes are also significant so in
.0006 it would have 4 sig figs
Trailing zeroes are not significant so in 400 it only has 1 sig fig
In your number 1.23456789 to shorten it to 3 sig figs it would be 1.23