The answer to your question lies in the naming of these compounds.
3-Ethyl hexane is the correct name for the alkane with a continuous chain of 6 carbon atoms, the 3rd carbon atom containing an ethyl group (C2H5) as a substituent.
The total number of carbon atoms is 8, so the molecule is an isomer of octane, but the longest continuous chain in the structure contains only 6 carbon atoms, so it is named as a derivative of hexane.
The other two compounds in your question are incorrectly named and do not exist as such!
If you draw out their structures you will see that each has a longest continuous chain of 7 carbon atoms and is therefore named as a derivative of heptane. You will also see that on the third carbon atom of the continuous chain, there is an attached methyl group (CH3).
In fact these other two compounds have identical structures and have therefore the same name. They are both called 3-methylheptane, and this is an isomer of octane.