when settling/closing the leverage value has to be higher (or equal) than 2. If it is 2,3,4, or 5 does not matter cause closing/settling will "remove" all your positions to your set amount, regardless if it was leverage 2-5. That's why it prefills with leverage 5.
It seems you did not understand margin trading?
You write that you "dont't want to risk all your 3 ETH" and that's why you do margin trading, but thats just wrong. Even with margin trading you risk everything you are using for that position.
In your case with a 2 ETH short poisition with Leverage 2 , you are risking 1 ETH.
But you are paying quite high fee for opening and holding a position.
So it would be wise to just sell 2ETH without any position and make a stop loss. With help of stop loss you also won't loose more than 1ETH, so you have the same result, but are paying much less fees.
Margin trading is used if you want to trade more then you have. So if you would like to sell 6 ETH or more, but you only have 3.
If you want to sell/short 2ETH but have 3ETH, opening a position makes no sense
Now the use of closing/settling:
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Closing is reverting your position trade.
When you go short with 2ETH this mean you are borrowing ETH and sell them. When you now close this position it does mean you are buying ETH back (at the price you set) to pay the borrowed ETH back (and taking the profit/loss, so the difference)
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Settling is paying the borrowed funds back directly.
So instead of buying the previously sold ETH back, you give some other ETH from your wallet to the lender. So in your case, if you would settle the 2ETH short position, the endresult will be that you sold 2ETH at 100€ and have 1 ETH left now. The actual price does not matter when settling, since you are just giving your ETH to the lender.
When is settling useful? It is very useful if you have eg. 100 ETH in your coldwallet, but only EUR or any other currency at the exchange. Now you want to sell your 100ETH, but the network would take too long. So you can place a short position about 100ETH eg at 100€/ETH, using your EUR in account as colleteral. And when your ETH arrives after x minutes/hours, the price went already down to 70€/ETH, you can settle this position, to really get 100€/ETH.