In the Copy Activity settings tab, there is a parallelism option. Is yours set to 3? That, in combination with the number of files written, would explain.
No matter what I change , upgrade the IR to 8 cores, upgrade the Azure synapse from DWU100c to 400c, change the degree of parallel to from 4 to 8 .. it still using '3' as the 'used parallel copies'
using Dynamic partition range . and the source is Azure synapse and the sink is ADLS gen2