Because 0 cannot come in the first place then 9 ways to fill first position, 10 ways for the next position and same way 10 ways for next position.
9*10*10=900 ways to have 3 digit numbers which dont start with 0
next there are 10 ways where first position is 1st and 2nd position numbers are 1 and 2(121,122,123,124,125,126,127,128,129) since this is the only way to get position 1 and position 2 1 and 2 respectively then position 2 and 3 we can get in 9 ways 112,212,312,412,512,612,712,812,912 so sum of that is 19 so 900-19=881 ways.