Is 3000 mg of garlic too much? I am cleansing my liver but I want to make sure it will have no side effects.

Liver cleansing??? How do you know your liver needs cleansing? And what about all the rest of us who don't "cleanse" our livers? Cleansing your liver is like cleansing your colon--neither is necessary and there is no valid medical basis for either. That much garlic is probably not a problem (emphasis on probably), but cleansing your liver is definitely not needed (emphasis on definitely)! you're young and healthy!
It depends. This depends on the form you're using. The world health organization recommends 2-5g of fresh garlic, 0.4-1.2g of dried powder, 2-5mg of oil, 300-1, 000mg of extract, or other formulations corresponding to 2-5mg of allicin (a constituent for which garlic is standardized). So if you mean fresh garlic, you're within these limits.