Hint:
let $\lim_{n\rightarrow \infty}x_n=l$
Then from your equation $$ l=-\sqrt{3-l} $$
hence $l^2=3-l$
solving we get
$$ l=\frac{-1\pm \sqrt{13}}{2} $$
I have a feeling, you should go for $l=\frac{-1- \sqrt{13}}{2} $
as every term of sequence is negative.
Where did I use the initial condition?
I dont know, if it is even required.