Is it possible to flash CM for Samsung S3 I9300 on different version? Is it possible to flash CM on a different version? The other one is a SCH-I939 which was bought in China. Can someone explain to me how I can do it and why I can't flash it? I flashed CM13 on the SCH-I939 but it shows not SIM card. The sim card is a US one and works on other phones so there is definitely no problems with it. 

Thanks in advanced!
Simply no.

i939 is a China Telecom variant, and has a CDMA baseband, along with other minor differences (there's a variant of i939 itself that even differs in dimensions). Unless the build was specifically marked as compatible with multiple variants (e.g.  S3 Qualcomm versions, aka d2lte, share a single CM build), the build will be limited to that model only. It's even more obvious considering i9300 is a GSM-only device.