Let's assume that user has a wallet with 3 bitcoin addresses: A, B, C.
The user received 1 BTC to address A, 1 BTC on address B, 1 BTC on address C.
Now the user wants to send 3 BTC to another user. How would he do it? What about miner fee, will it be more than in case of sending from one address to another? Please help me understand that question.
How would he do it?
It depends on the wallet manager you are using.
If you mean how does it do it under the hood it adds multiple inputs to the transaction and signs it with the private key of each address.
What about miner fee
Considering that the suggested fee should be proportional to the weight in terms of data of the transaction, since a transaction from multiple addresses should be smaller (always in terms of data) of multiple separate transaction the fee can be slightly smaller too.
Yust to give you an idea writing
A -> D
B -> D
C -> D
Takes more space than writing
A, B, C -> D
