Got reply from STMicro forum.
"PB3 SWO is typically used in support of the SWV (Serial Wire Viewer), as far as I'm aware the Cortex-M0 does not support this functionality (Debug/Trace), or JTAG, so the pin can be left unconnected.
If may be connected in the schematic due to the way ST shares a common footprint over several processor families, allowing migration up and down the feature/power/cost line.
I, personally, would opt to use the standard 10-pin 0.05" pitch ARM programming header over the ST ISP one."