For $f(-1)$, since $-1<0$, we use the first equation $x^{2}+3$. So, $f(-1)=(-1)^{2}+3=4$.
For $f(0)$, since $0$ is not less than $0$, we use the second equation $3-x$. So, $f(0)=3-0=3$.
For $f(1)$, since $1$ is not less than $0$, we use the second equation $3-x$.
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