Question

How do I write a system of 3 equations that has a unique solution (1,2,3)?

Answer 1

Do you want a system of lines or a system of planes?

Answer 2

I really don't know where to begin. I guess a system of lines.

Answer 3

The simplest system of equations would be
\[\begin{cases}
x=1\\
y=2\\
z=3\end{cases}\]