3 long beeps Intel Motherboard?
Answers:
Time to go back to the drawing board and troubleshoot your system. Since you have replaced the RAM with "good" RAM we have to assume that the problem lies somewhere else.
First ask yourself did I put anything into the computer (new cards and such) If you did remove them and try again. Usually I remove all cards and just connect the keyboard mouse and video up to the system. If the system starts then you know its one of the cards you have removed. If you still are in a no start senario then the problem lies either in the RAM (may not be complatible with this board although is good on the other machine), CPU or the Motherboard itself.
Sometimes the RAM slots on a MB break free during memory removal. (The MB is basically toast then unless you're really good with a small soldering iron) Check the slots to make sure the wires aren't bent and touching each other or 2 leads on the RAM stick when you place RAM into the slot.
A coulple of other questions/routes to investigate:
Check ALL Cables and Connections, including the panel wires for the reset button, HDD LED and such.
Recent electrical storm?
Mice? Yes the get into the computer where it is warm, do their bathroom business and short out the board.
Cockroaches? They eat the insulation on the wires and the motherboard.
I have seen it all in my 25+ of computer repair.
It could be that whilst you were checking your ram something might of come lose try checking all the wires and making sure they are all pushed in
Make sure that your ram and your video card are completely seated.
Try all the sockets.
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