[1] The third party is considered independent from the other two, even if hired by them, because not all control is vested in that connection.
There can be multiple third-party sources with respect to a given transaction, between the first and second parties.
A second-party source would be under direct control of the second party in the transaction.
[2] In information technology, a third-party source is a supplier of software (or a computer accessory) which is independent of the supplier and customer of the major computer product(s).
An example of 3P sellers are merchants participating in Amazon's FBM program.