The basic sketch is as follows:
This is a half-plane in the coordinate plane (x,y)(x, y)(x,y)
The boundary line is x+y=3x + y = 3x+y=3, inclined at 45°, passing through the points (3,0)(3, 0)(3,0) and (0,3)(0, 3)(0,3)
The solution region is below this line, excluding the line (because it is “<”, not ...