C# 3/A#
The C# 3 chord consists of the C# (Perfect unison) and F (Major third) notes, with A# as the bass note.

How to play the C# 3/A# chord
- Play A# as the lowest bass note in the left hand
- Find C# (Root) on the piano keyboard
- Add F (Major third) — 4 semitones above C#
- Press all 3 notes simultaneously to sound the C# 3/A# chord