\(\lim\limits_{x\rightarrow+\infty}\left(x-3\right)\left(\sqrt{x^2+x+3}-\sqrt{x^2+x+1}\right)\)
=\(\lim\limits_{x\rightarrow+\infty}\left(x-3\right)\frac{x^2+x+3-x^2-x-1}{\sqrt{x^2+x+3}+\sqrt{x^2+x+1}}\)
=\(\lim\limits_{x\rightarrow+\infty}\left(x-3\right)\frac{2}{\sqrt{x^2+x+3}+\sqrt{x^2+x+1}}\)
=\(\lim\limits_{x\rightarrow+\infty}\frac{2\left(1-\frac{3}{x}\right)}{\sqrt{1+\frac{1}{x}+\frac{3}{x^2}}+\sqrt{1+\frac{1}{x}+\frac{1}{x^2}}}=1\)