You can try to slow your memory timings down. Often, with large memory loads, you can't run as tight timings.
What I did on one of my rigs was to slow the timings way down, add the extra memory, then gradually bring the timings back up to see how far I could get with 1.5 Gb installed.
If you look at memory configuration table on page 21 of the manual, you will see that this motherboard only supports 1, 2, or 4 memory modules (not 3).
You are about to answer a thread that has been inactive for more than 6 months. If you still wish to proceed, please ensure that your posting is original and does not duplicate or overlap any prior responses to this thread.