Hi,
When a port is configured as a SPAN destination port, the configuration overwrites the original port configuration.
if all your ports are on the same switch, then you can try the following commands.
eg. source ports 1/1(vlan 2) , 1/2(vlan3) & 1/3(vlan4)
Destination port 1/40
monitor session 1 source interface fa 1/1, fa 1/2, fa 1/3
monitor session 1 destination interface 1/40
refer the following documnet for configuring SPAN on 3750 switches
http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_guide_chapter09186a00801cdee2.html
HTH
Narayan