Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
Those 3 beeps with 5 second pause indicates that there is some issues with the RAM memory. It is possible that either one of the RAM module or both RAM modules are failed somehow. You may need to buy a new RAM memory.
Refer this article --> https://support.apple.com/en-in/HT202768
So I want you to take out both modules and test with one RAM module at time. This will help you to rule out failed memory module.
Refer this article to know about replacing the RAM --> https://support.apple.com/en-in/HT201165
Please let me know if that didn't work, so I will continue to help you
Note : You may get an offer regarding secure remote session. It is the offer sent by this site. Since your Mac is not booting up, I will not be able to connect and fix it from here. So please ignore it for now.
Try the following steps then, this might not work but it is worth a try.
First reset the System Management Controller of your Mac. Follow the steps below;
Shut down the computer.
Plug in the power adapter to a power source, connecting it to the Mac if not already connected.
On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
Release all the keys and the power button at the same time.
Press the power button to turn on the computer.
If that didn't work, then reset the PRAM and NVRAM of your Mac. Follow the steps below;
Shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac boot up.
If it still not working, then you will have to replace the RAM. You can find the instructions in this article to know how to remove the memory --> https://support.apple.com/en-in/HT201165
There is nothing wrong with other parts of the Mac. It is with the RAM memory of the Mac. Try it. It is pretty easy and user removable.
Let me know how it goes
See this sample Youtube video with instructions --> https://www.youtube.com/watch?v=6Lmp_rwtJg8
or this one --> https://www.youtube.com/watch?v=WvMeVtU6RcM
It is easy to remove the memory. You will find two memory modules there. Just test it with one at a time and this will help you to identify the failed one.
That means one or both of the memory modules failed. You need to open the back cover and remove one. Then test with one at a time. Click this link and you will find the instructions --> https://support.apple.com/en-in/HT201165
Or See this Youtube video with instructions --> https://www.youtube.com/watch?v=6Lmp_rwtJg8
This is not a software issue, but hardware related. This is not a fatal failure. You just need to replace the RAM and RAM starts from $20 onwards.
If you are not comfortable replacing the RAM memory yourself, then you can take it to the Apple Store. You will not lose any files from your Mac. RAM memory starts from $20 and more for high capacity ones (RAMs are the small memory sticks of your Mac).
It may cost more if you replace it from Apple Store. I hope I've provided the answer for your question. I'm pretty sure you're looking for a solution but I'm sorry, this is an hardware issue and In this case the only solution is to replace it. If you are not satisfied, please don't rate me bad or poor as it will badly affect my job here.
If you have any questions, please let me know and I'm happy to answer all follow up questions.