This is basic plucking, or, the Staunton Lick.

Hold down the chord of C, pluck a fretted bass string with the thumb of the right hand.
[ Lyrics from: http://www.lyricsfreak.com/l/lemon+jelly/the+staunton+lick_10163985.html ]
And pluck the third, second and first strings altogether with the first, second and third fingers of the right hand.

This is the Staunton Lick.

The Staunton Lick.
The Staunton Lick.