The B flat diminished chord is a 3-note chord consisting of the notes Bb, Db and Fb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Bbdim.
Chords that are a superset of Bbdim. The chords include more notes but always Bb, Db and Fb.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Bbdim chord. Feel free to save or share the image as needed.