Method to solve an equation

The question is: put one of $1,2,3,4,5,6,7,8,9$ in one position below, to obtain $100$
$$largeoverline{squaresquare}+overline{squaresquare}+overline{squaresquare}=100$$his mean fint
This mean to find three,two digit number with sum of 100.
I can find by guess and exam some of the possible answer like below
$$45+37+18=100\
46+35+19=100\65+23+12=100
$$ But I am looking for a $largetext {method}$ to solve the equation.
AS a second question, how to find all the possibilities? (when do not repeat any number)
Thanks in advance for any help.