三个原子在一起,先不要管它们是不是排成一条线. 把HCN看成三角形.
------C----
-----/-\---
----/---\--
---H-----O-

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Vibration Mode--1: H---C vibration.
------C----
-----/-\---
----/---\--
---/-----O-
--H
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Vibration Mode--2: C---O vibration.
------C----
-----/-\---
----/---\--
---H-----\-
----------O

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Vibration Mode--3: H---O vibration, or called bending
------C----
-----/-\---
----/---\--
---H->-<-O-

in which H and O atoms will move toward or away to each other.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Vibration Mode--4: off-plane vibration.
The equilibrium position of C atom, H and O will keep in the same plane. only C atom vibrates up and down from this triangle plane.

-------^-----
-------C-----
-----/-|-\---
----/--V--\--
---H-------O-

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-----
Finally, I think the linear structure is very misleading. Triangle could simplified the problem.
best regards.