Hey ye 3750s only support netflow in x series , they need a specific module too , standard 3750s don't support it as you seen
only other available commands I can think of may be ip accounting , or RITE if available on your ios version depending on what it is , you can span or rspan the ports either to wirehshark and just set the captures to be an hour long each that way if you something's happens at a specific time you can check the individual pcap based on the time.
there not ideal options but without netflow your limited
