Three Beeps At Startup

3 beeps is a RAM issue.

Try re-seating the RAM: MacBook Pro: How to remove or install memory - Apple Support https://support.apple.com/en-us/HT201165


you are testing for both bad RAM or bad bank—

one stick at a time in each bank, If the RAM sticks prove to work individual—and banks work individually.


Other known issues are: 

worn clips, or RAM banks slightly distorted (out of flat)

Using shims to secure broken/failing clips on RAM slots shims

Yes it seems like the out of flat distortion or a broken/failing clip could well lead to this symptom you are seeing— if moved from a stable, flat surface—any pressure on the bottom cover could upset the RAM contacts—leading to the error.

If you have isolated to bad stick of RAM— it maybe a chance for you to upgrade. the 2012 can manage 16GB of RAM in total. Best in matched pairs and to buy RAM from reliable source with the exact specs.


Usually a lifetime guarantee on RAM from third party source like:

https://eshop.macsales.com/shop/apple/memory/

http://www.crucial.com/usa/en

When reseating the RAM as leroydouglas suggests, make sure they are snapped/locked in place. Sometimes the first module can become unlocked when installing the second module so you may need to use a toothpick to gently press on the edge of the upper module nearest the keyboard to verify it is locked in place. Just make sure to press on an empty area so not to damage the very small surface mount components. Check to make sure the little triangular protrusion on the locking levers is able to lock each RAM module in place.

Disconnect the power adapter & battery while inserting and removing memory and other items.

I've also seen issues with the lower slot (the one nearest the Bottom Case). If pressure is applied to the center of the Bottom Case or to the black base of the memory slot it can cause memory loss or system freezes. Sometimes gently pressing on the lower memory (nearest the Bottom Case) can cause it. Sometimes reseating the memory works, but other times I've had to remove the "cushion" from the Bottom Case so it no longer presses on the RAM. If the "cushion" is removed, electrical tape needs to applied in place of the cushion to protect the memory from a short if the bottom case manages to bend in far enough to touch the memory. Sometimes the only option is to remove memory from the lower slot (closest to the Bottom Case).

RAM soldered to the board,,rather than in a socket, is more reliable. That is the main reason Apple has moved to soldering the chips to the board.

You have one of the last portable models with RAM in sockets. As leroydouglas says, It will accept up to 16GB -- up to an 8GB DIMM in each of its two sockets. DIMMs can be replaced by the each DIMM. Performance with a matched pair is very slightly faster, but not noticeable.

For reasonably-responsive operation with El Capitan and later, you need at least 6GB on board.