Re: Bitfinex. I'm in. Why are there three different Bitcoin deposit addresses?
Exchange Wallet
Trading Wallet
Deposit Wallet

You should ask them, anyone can troll you or give a honest but wrong answer. The explanation must be somewhere in the site, and you can contact their support
The Bitfinex support page explains why are there three different Bitcoin deposit addresses. It's because there are three different account types. You can move your Bitcoins over to any account type you like at any time you want on your account Settings page. If you only want to buy and/or sell Bitcoin open an Exchange account and use the deposit address for that.
https://www.bitfinex.com/supportWhat are the three account types and which should I choose?
Trader: This account type allows access to margin trading, exchange and margin funding
Exchange: This account type allows access to exchange and margin funding
Deposit: This account type allows access to margin funding
Can I change my account type later?
Account Settings page
How can I take advantage of Bitfinex's features even if I don't feel like buying any Bitcoin or Litecoin?
You can open a Deposit account and provide margin funding in US dollars to other Bitfinex traders. See Margin Funding.
I would like to simply buy and/or sell Bitcoin, what should I do?
You can open an Exchange account and use the "Exchange" tab of Bitfinex