Hi influx. It is not really correct to state anyone of these resistors to be in series with another one here.
It often is clearer if you think in terms of parallel and series paths. If you can find a route that allows current to flow from one end of a resistor to the [node at the] other end of that resistor, and without passing through that resistor, you have found a parallel path.
If all current that passes through a particular resistor must also pass through another resistor (and vice-versa) then those two resistors are in series.