Startup problems
Older Macbook Pro. Three beeps repeating. Mac fails to startup.
MacBook Pro 15", macOS 10.13
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This would be RAM problem.
You do not say what exact Mac /model /year /size /retina /touchbar is this
or
what exact macOS you are currently running?
For example 2012 MBP...
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:
or
RAM banks slightly distorted/out of flat (use shims)