f(x)=[3x]-[x].

f(0.9)=[3x0.9]-[0.9]=2-0=2.
f(0.99)=[3x0.99]-[0.99]=2-0=2.
f(0.999)=[3x0.999]-[0.999]=2-0=2.

f(1.1)=[3x1.1]-[1.1]=3-1=2.
f(1.01)=[3x1.01]-[1.01]=3-1=2.
f(1.001)=[3x1.001]-[1.001]=3-1=2.