The pure quantum state that satisfies your conditions is the W state in three qubits,
$$ \frac{1}{\sqrt{3}} \left(|001\rangle + |010\rangle + |100\rangle \right) $$
You can look at this answer for a high level circuit to construct this. The first gate in that circuit is a single qubit gate that effects the transformation,
$$ |0 \rangle \rightarrow \frac{1}{\sqrt{3}} |1 \rangle + \sqrt{\frac{2}{3}} | 0\rangle .$$
This you can implement in the composer as a $U_3$ gate with an appropriate value of theta.
Next you will need a controlled H gate between the first and second qubits, and a Toffoli gate. To implement them in the composer you can use the circuits given here
. The control gates in the answer have the control qubits flipped (the controls are triggered by $0$ and not $1$ ). So you will need to sandwich your control qubits in the composer between $X$ gates to get the desired circuit. As you can see, constructing this from scratch in the composer is rather tedious.