thanks to all, folks!!!
at last i've got it.
the principal issue was i only get 1 of 3 private keys.
i've to got the private key from trustedcoin in order to get 2 of 3.
and at the end i've understand what all you wanted transmit to me.
thanks again for your support.

please help with btc multi-sig address (starting with 3)
from tenx team support inform me that some people who make their transactions from a btc multi-sig address (starting with 3) have found the way to sign the message we need to prove the ownership of the address.
i don`t know how can i do it.
please can anyone help me?
thanks thanks thanks

thanks @ranochigo and @Muhammed Zakir
i'm closer to the solution
following your indications i've got the master private and master publics keys the multisig was created and now i've a multisig 2-3 (not a 2FA)
in order not to bother you, i'm trying to follow next step by myself...but i give up!!!
sorry to write you again... can you help me how can i do to follow your next indications???
thanks thanks

@ranochigo "sign a message with the addresses in the public key that is used to create the multisig. As long as you can get at least n signed messages in an n-of-m configuration, it is sufficient proof that you control the address"

@Muhammed Zakir "generate address with it and sign a message. According to ranochigo, you need to sign message with an address created from each of the private keys"

Thanks and sorry for my ignorance... please be in mind I'm a rookie...sorry!!!

I give you more information in case you can continues helping me:

My address starting with 3 is a Trustedcoin 2FA wallet generated by Electrum wallet. Therefore I'm not sure I've got the public key that was used to create the multisig. What I've got are the 3 publics keys and the private key correspondign to my address starting with 3.

I think is a 2-of-3 configuration (one of them is owned by Trustedcoin in order to make the 2FA process; and the other 2 it's supossed are mine) And the matter it's that I don't know how to get the 2 signed messages you told me to proof that I control my address starting with 3.

Please if you can be more detailed for me I'll be grateful!!!

For multi-sig-addresses (a subset of P2SH addresses) there are private keys, but there is no consistent definition on how a signature for a message would be verified. Thus, it's impossible to sign a message with an address starting with a 3 in a reliable way.
Therefore, I'm not able to prove via signing a message that a address starting with 3 belongs to me.
Even when I import the private key that Electrum give to me into another wallet, the address imported is different, starting with 1. Notice that I can sign message with this new address (starting with 1), but obviously the message is not linked to my original address starting with 3.
Please, anyone can help me??? Thanks a lot!!!