They are the same rifle, it's just stamped with a different model number to get around U.S. government infringements on our right to bear arms. However, the SAR-3 and SAR-8 overstamped are preban rifles, so they are better because then you don't have to deal with more U.S. government infringements which require you to add 7 U.S. parts when replacing the fake U.S. government required flash suppressor and thumb hole stock.
In summary, all varying model numbers are due to our government infringing upon our right to bear arms. Here are the best to worst, but they are all essentially the same rifle:
SAR-8 overstamped on SAR-3
SAR-8 (post ban) - U.S. government requires 922r compliance to swap parts.
SAR-8 (US D.C. Industries junk aluminum receiver) only buy for parts, $500
Nice progression from G3 to junk, eh?
Thanks to our representatives violating our supreme law.