I think that you are being confused by poor wording.
If there are three resistors in series in a circuit, the current flowing through each resistor is the same.
If we have a 10 volt supply, connected to a 2 ohm, a 3 ohm and a 5 ohm resistor in series, the current flowing will be 1 amp. The current is the same in each resistor.
If we now add a fourth resistor into our series chain, the current through each resistor will be the same, but it will not be 1 amp.
If the fourth resistor is 10 ohm, the current will now be 0.5 amp.
Does that help?
JimB