NPSH

Check a few of these sites.

If the flow rate decreases, and all other parameters (static head, pumping distance, temperature, atmospheric pressure) all remain the same, the NPSHR (required) will decrease as the flow rate decreases. The reason for this is the only variable in the equation: friction losses. The decreasing flow will have a corresponding decrease in friction loss. This will cause a corresponding decrease in the total dynamic head, both on the suction side and the discharge side of the impeller and volute.

If you will observe the NPSH curve on the pump characteristic curve, you will see that the required NPSH increases as the pump flow increases. Therefore, the available NPSH will decrease as the flow increases, again, because of the increase in the total dynamic head the pump is required to overcome.

I don't understand why you are experiencing a decrease in available NPSH when the flow rate decreases, or am I not understanding what you are telling me? It should be the opposite. The required NPSH should increase as the flow rate increases.

The 3% head drop while testing the pump doesn't mean anything to me. Perhaps this is a characteristic of the pump manufacturer's testing procedures.

I don't understand why you are experiencing a decrease in available NPSH when the flow rate decreases

I think the OP meant to say when the flow rate increases. He wondered why the available and required NPSH curves have opposite curvature as seen in the diagram above.

I'm not sure about the 3% reference, but the following excerpt from this source follows:

In pump operation, two aspects of this parameter are called respectively NPSHA or NPSH (a) Net Positive Suction Head (available) and NPSHR or NPSH(r) or NPSH-3 Net Positive Suction Head (required), where NPSH(a) is the suction pressure presented at the pump inlet port, and NPSH(r) is the suction pressure limit at which the pump's total differential head performance is reduced by 3% due to cavitation. It's important to note that cavitation occurs at suction pressure levels above the NPSH-3 level and pump damage can occur from cavitation even though the pump may continue to provide the expected hydraulic performance.