Hello, i have a pc that uses ddr3 ram, yeah yeah i know its old, anyways so recently one of my ram sticks died so i bought two used ddr3 ram sticks from a guy, so when i put the ones he sold me and power on the pc the pc beeps 3 times and theres no signal on my monitor. the beepinig only happens when i put the ram modules i bought from that person and my original ram modules are out of the system, ive also tried putting them one by one and still both of them give me the same result either when they are both in the pc or just one, when they are all in the pc with my original modules on the pc powers on but no signal to monitor, ive done some research on the internet and ive mostly have found that 3 beeps means that the ram modules could be dead or may have a problem, but is there anything else that could be the problem? could it be different ram speeds or if the ram ive obtained have faster ram speeds could it be that my motherboard wont work at those speeds? the pc im using is a dell optiplex 990