Is Oregairu Season 3 the end? I'm wondering if season 3 of Oregairu were the end of the light novel because I intend to read the light novel of it if it wasn't the end.
Is season 3 the end of the light novel?
There is a short sequel released alongside the first BD.
Wataru Watari, Oregairu's author, confirmed via a tweet that a six-episode bonus novel titled My Teen Romantic Comedy SNAFU -Shin (New) will be released alongside the first volume of the BD. The BD's release was on September 25, 2020. Furthermore,

...the bonus novel will be a completely new orthodox sequel story to tell &quot;new days.&quot;