I created 3 different addresses with the same private key.
That seems like a silly thing to do. Why did you do that?
1 BTC was sent to each of these addresses.
I will send 3 btc to another address.
If you are planning on using the 3 BTC that you received at the three addresses, you'll need some additional bitcoins for the transaction fee.
Can 3 BTC be transferred at once
Using a single transaction? Yes. Transactions can have multiple inputs. You can create a transaction with 4 (or more) inputs, one for each of the 1 BTC received, plus one (or more) for the value being used for the transaction fee.
with the same signature?
No. Each input will have it's own scriptSig (also known as TxIn-script)
The sending address appears on the network.
This is a VERY common misunderstanding about how bitcoin works. There actually isn't any such thing as a "sending address", many block explorers will attempt to decode the UTXO and assign a "sending address" in their interface. This common practice expands the confusion and misunderstanding that people frequently have about how bitcoin works.
Instead, you should think of a bitcoin address like an "invoice number". It allows you to determine when someone has paid you, and how much you got paid. If you do the smart thing and give out a brand new address for EVERY transaction, then you'll know EXACTLY who paid, when, why, and how much.
While a business will receive payments on invoices for their products and services, when they later spend those funds they'd never use the phrase "sending invoice" to indicate which invoice was originally used to receive any particular funds that they might be sending to someone. Equally, it's inaccurate to use the phrase "sending address".
Yes you can send out all the transaction at once’s but it will go as 3 output into one input
You've got that backwards. There will be 3 (or more) INPUTS of value into the transaction, and that value will then be asigned to the 1 OUTPUT where they want to send the funds.
requires everything you hold on that wallet it will all be sent at once
It's never "required" for everything held by a wallet to be sent at once if those funds are spread into multiple UTXO. Most wallets will try to do a good job of choosing a subset of inputs.
the only thing their is that the transaction size will be larger compare to when you are just sending from 1 output.
Definitely. Funding a transaction with a greater quantity of smaller value inputs will result in a larger (in terms of bytes) transaction than using a smaller quantity of higher value inputs. Since fees are paid per byte (or rather per vByte), this will generally result in higher overall transaction fee for a comparable smaller (bytes) transaction sent at the same time.
Which sender address is visible?
As I stated earlier, there isn't really any such thing as a "sender address". This is a VERY COMMON misunderstanding about how bitcoin actually works. What exactly are you trying to ask here?
Are you asking if the inputs to the transaction can be identified? Yes, the inputs to a bitcoin transaction can ALWAYS be identified. Thay HAVE TO be. That's how Bitcoin nodes are able to verify that the transaction is valid without needing to trust any peers or authority.
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Well said. I have nothing to add here. Thank you for your excellent contribution. Merits sent.
I mean the 3 wallet which send the 3 bitcoin
It is generally not possible to determine what "wallet" was used to send a bitcoin transaction. Occasionally, some wallets will be unique enough in how they construct transactions that it might be possible to infer the wallet that was used, but that isn't common. Typically it would be impossible to determine if the transaction was sent with Bitcoin Core, or Electrum, or Blue Wallet, etc.
will be visible as the output wallet (sender address)
Output wallet? sender address?
I'm going to guess that what you're trying to say is:
It will be possible to determine the UTXOs being spent in the transaction. Once you've identified those UTXOs, it will be possible to determine the scriptPubKey (also known as the Tx-Out-script) in that UTXO. From that scriptPubKey, it will be possible to identify the public key hash that was used. With the details of the script and the public key has, it is possible to generate the "address" that was used to create the transaction where the funds were previously received.
Your wallet displays them as
How his wallet displays things will depend on what wallet he's using. I don't think he ever indicated in this discussion what wallet he uses. Did you determine that from some other conversation you had with the OP?