fly to Croatia on Friday, trying to figure out, how much US dollars, Euros, Kruna, 3 week trip

Croatia went onto the euro at the start of 2023, so you no longer need to get Kuna.

Dollars are not accepted; only Euros. I would suggest waiting until you get there to get Euros. Most seasoned travelers use credit cards as much as possible, but it's always good to have some cash available. Wait till you get there, then find an ATM at the airport and request $100, which should probably last you a bit..

If that's not enough or you run out, you can always get more from another ATM. But changing money while you're still in the states will be much more expensive, as will exchanging money at an exchange kiosk at the airport. Stick with ATMs if possible.

Make sure you notify your banks that you are traveling. I suggest you do not use a credit card to withdraw money, as that is often considered a cash advance with a high interest rate that would offset any points you might earn. Using an ATM or debit card (with a PIN) is the most efficient way to get cash, but look at the details of your account to see what kind of fees you may incur there as well. Some cards don't charge a foreign transaction fee but there are many that do. If you're worried about having cash on arrival, as you're in LA you probably have more than one option (local bank or AAA office) that you can purchase Euros today.

And use a regular bank’s ATM to get cash with your credit/debit card. You don’t want to get gouged by the
Colorful “Euronet” ATM’s that seem to on every street corner in Dubrovnik, Split and Hvar and elsewhere in Europe. Euronet is a private company with 50,000 ATM’s that charge outrageous rates and hidden fees.

EuroNOT.

You will likely find out that you will use very little cash. Credit cards are used most often. If you get cash in advance (I wouldn't, I would get money upon arrival as outlined in previous comments) you should need 100 euros, at most.

Use an ATM as needed.