I did. I think I've tried to send those money for like 20 times or so. And always either got tx broadcast timed out error or no error, but transaction didn't apper at blockchain.info.
Which versions of Armory and Bitcoin Core are you using? Can you also post the armory log files (armorylog.txt and dbLog.txt)?
Well, I control addresses that I've explained in 2 and 4, but I don't control both addresses in 3. The first one that receives 0.007 BTC is unknown to me. The second (which receives ~3BTC ) is listed in my Armory's change addresses. Does that mean that I control it?
Yes, you control that address as it is a change address.
Did you attempt to make a transaction of 0.007 BTC since April 25th?
Are you sure there's nothing can be done about the situation and I will basically lose money if the transaction from 3 will be confirmed?
Since you control the change address, you will at worst lose 0.007 BTC.
Is there a possibility to double-spend it once again with like 10x fee maybe?
Yes, but I wouldn't recommend it since you already have multiple conflicting transactions.
No, not bad at all. That site uses Bitcoin Core in the background and if it receives a transaction which conflicts with another in its mempool, that conflicting transaction will be rejected. What happened here is that it received the 3rd transaction first thus it rejected and did not keep the other two transactions.
Also. If we assume that my PC is compromised, is there anything I can do right now to prevent the hijacker from creating new transactions from my wallet?
If they have a copy of your wallet file and the password, then there is nothing you can do. However, I think it is unlikely that your computer is compromised. Rather I think the issue lies in Armory doing something wrong because of multiple broadcast failures.