How to send btc from multiple addresses to a single btc address? Let's say, I have 3 btc addresses A, B, C i.e; 1 BTC in address A, 1 BTC in address B, 1 BTC in address C. Now I want to send 3 BTC to another address. How to do that and what about the miner fee?
If you were asking about creating transaction manually, you can do that by creating raw transactions using addresses (A, B, C) inputs.

You can set the fee by not spending the fee while creating the transaction.
so if you have 3 BTC you send 2.998 and just leave the rest, it will be considered as miners fee.  

By default if you are using Bitcoin-core or any other wallet they are using required inputs to cover your payment, so you shouldn't worry about that.