Question 1085052
Sometimes. 
{{{f(x+3)=f(x)+f(3)}}}
When {{{f(x)=0}}} then,
{{{f(0+3)=f(0)+f(3)}}}
{{{f(3)=f(0)+f(3)}}}
{{{0=0+0}}}
{{{0=0}}}
True.
.
.
.
Sometimes not,
{{{f(x+3)=f(x)+f(3)}}}
When {{{f(x)=x^2}}}
{{{(x+3)^2=x^2+9}}}
{{{x^2+6x+9=x^2+9}}}
{{{6x=0}}}
False.