memory trouble

[xwrench3]

3 years ago, my son and i built him a gaming computer. it was both of our first computer build, which made it pretty nerve wracking, but we took our time, and in the end he had a fully functional hot rod gaming computer. anyway, he came to me last week saying that the newest game he is playing needs more ram. we built it with 8gb (2-4gb modules), so we looked up the specs of what he had, and ordered up a single 8gb module that otherwise matched the already installed memory. so, it came today, and he grabbed it and ran off to his room to install it. but, something went wrong along the way. the original ram is installed in dimm A2 and B2  (ASUS Z97-A mobo), he installed the new stick in B1, which is the easiest slot to access, because of the CPU cooling fins covering up the A1 slot. when he tried to start the computer, it would not start.  so of course, now he wants my help. in the process of trying to diagnose the problem, he told me that he had not turned off the power supply, and the computer was actually asleep, not powered down. in the process of trying to get the computer to work, i removed the 4gb module in dimm B2, and when i powered the computer back on, it came to life, reading 12gb of memory. i tried powering it back down, and reinstalling the 4gb module, but the compter will not start again. he could not find the "owners manual" that came with the motherboard, and the one i found online has no recommendation on placing a 3rd memory module, just 2 or 4. is there a specific way to place the modules to make them work in this arrangement (3 modules)? or did he fry a module? his is the only computer in the house that uses DDR3, so we can not test it by trying it in another computer.