Using axioms to define metric spaces

Using axioms to define metric spaces

Let $M$ be a set with three elements: $a, b,$ and $c$. Define $D$ so that
$D(x, x) = 0$ for all $x, D(x, y) = D(y, x) =$ a positive real number for
$x \not= y$. Say $D(a, b) = r, D(a, c) = s, D(b, c) = t : r <= s <= t$.
Prove D makes M a metric space iff $t <= r + s$.
I have no idea on how to begin this proof. I've taken multiple stabs at it
but to no success.