Just want to confirm is the 3750-12S GigE Port support variable speed or is it fixed 1GigE only.
Only the GLC-TX will support 10/100/100. If you want the SFP to support 100mbps (only) then you need to look at the following link:
Cisco 100BASE-FX SFP Fast Ethernet Interface Converter on Gigabit SFP Ports
The Cisco 100BASE-FX SFP on Gigabit SFP ports is supported on Cisco Catalyst 3750, 3560, and 2970 series switches.