Yes, you can simply use one output connector from the LPS to power your Volumio Rivo+.
Using only one output does not reduce the effectiveness or performance of the LPS in any way.
In fact, from a high-fidelity perspective, we generally recommend one linear power supply per device. While the interference between two devices powered from the same LPS would be extremely small, in high-resolution Hi-Fi systems it can still be perceptible. The dual outputs are mainly provided for flexibility, for example if a user wants to power two low-current devices. Using only one output is perfectly fine.
That said, since your Rivo+ requires 5V / 3A, we would actually recommend the LPS-35, which can provide 5V / 4A.
This gives more current headroom, which is generally beneficial for stability and sound quality.