what is the problem when it’s beeping three times and then pause for five seconds?

If your Mac beeps during startup - Apple Support explains,

Three beeps, then a 5-second pause, repeating

The memory in your Mac didn't pass an integrity check. If you added or replaced memory, make sure that it's properly installed.

Three beeps indicates a memory (RAM) problem. Hardware problems need to be evaluated by Apple Hands-on.