smc

Let us represent the increase or decrease in $x$ by $(x\pm a)$ Thus our original expression becomes $$(x\pm a{)}^2-3$$ $$x^2\pm 2ax+a^2-3$$ The absolute difference between these two expressions is $\pm 2ax+a^2$. Therefore, the answer is $\boxed{\pm 2ax+a^2}$