I've been working my way through the code for memtest86+ v1.65 - I added a smidgen of code to enable 975X support and have been looking at a couple of the issues that I found.
Which version are you using? v1.65 from this site?
http://www.memtest.org/
A few more details, if you please: which chipset, which motherboard model, which memory type/freq/manufacturer, what voltage on the memory? Are you overclocking? Is the memory settings in the BIOS set to auto from the SPD, or are you setting it manually?
Test 3 is an 8-bit memory inversions pattern - essentially walks an 8-bit "1" through memory. It catches "wide" errors - as opposed to single bit errors. A single bit error is often associated with a manufacturer problem, but if you pass test #2 (a single bit walking inversion), then a "wide" memory problem would often associated with memory timings or voltage issues.