The 3rd parameter should be [account_1, account_2, account_3], it missed the account_3. If you check other language examples, they all contain account_3.
The python example in this page:
also have this problem:
threshold = 2 # how many signatures are necessary
msig = transaction.Multisig(version, threshold, [account_1, account_2])
print("Multisig Address: ", msig.address())