My Mac is beeping 3 times every couple of seconds when it starts
That does sound like "Three successive tones, then a five-second pause (repeating)", which according to Apple, suggests that the RAM has not passed an integrity check.

https://support.apple.com/en-us/HT202768

You mention a disk slot, so it's likely quite an old Mac. The last models with optical drives were c. 2012, I think.

Either way, it's a hardware problem that needs hand-on, physical inspection by someone competent. See if there's a local guy who does Apple repair work. (The Apple Store may say it's too old, but it's worth a try.) 

It may just need the existing RAM to be reseated or replaced. Or it may be a problem with the main circuitry: in which case it's time to go shopping.