will the passphrase be required to use the existing Wallet within the multisig (to send, for instance)?
If you don’t choose to sign with the Trezor’s key, then no. Given that you have 2 master private keys out of the total 3, you don’t have to access the Trezor one, and thus, no need to type the passphrase. However, if you want to spend using a signature from Trezor, you’ll need the passphrase, because master private keys with a passphrase aren’t stored in the device.
Quoting wiki.trezor.io/Passphrase:
The passphrase is not stored anywhere on the device. It is only used temporarily whenever you enter it.
If I don’t want the passphrase to be required, but have funds on the existing wallet already, how would I go about moving the funds and creating the multisig with these 3 wallets, where no passphrase is required for that existing wallet?
Not sure I understand what you mean here. If you want to move funds from your 2-of-3 multi-sig (with the passphrase) to a 2-of-3 multi-sig (without the passphrase) you don’t need to access the Trezor, as I’ve said above.
