This printable PDF is a nice looking printout of the 4 main barre chord shapes you need to learn for Grade 3 guitar.
Each shape is moveable, so with these 4 shapes, you are able to play any major or minor barre chord on the guitar. For example, play the first major shape on fret 3 of the E string, and that will be G major, because the root note will be on G. The root note for each chord is shown on the sheet as black circles. So, if you want to play a C minor barre chord, use the bottom right chord on the sheet (the minor shape with the root on the A string) and match up the root note with fret 3.