3key is an experimental input method involving, you guessed it, just three keys.
3key is a chording keyboard, which means that typing most letters requires holding certain combinations of keys (chords) down in sequence (these sequences are called "codes").
These chords are all designed to differ from eachother strictly by requiring a new key to be pressed, or a pressed key to be released; this permits typing at any speed.
These codes are summarized in the reference sheet, which shows the individual keys as a row, and time along the vertical axis.
The Android version lives in this repository: https://codeberg.org/512b/threekey. Keep in mind however that no builds are provided yet; you will have to build it yourself.
The initial proof-of-concept demo was keyboard-only (uses the IOP keys) and can be located at demo.html.