Watch the G Chords online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners

Another great - and very popular - three-chord grouping is G, C and D. You know C and D already, so the G chord is all you need to learn. G is a 6th-string based chord; you can strum all the strings when you hit a G chord.
As in prior examples, we're using a three-chord progression in a 4-bar progression. Using shorthand, the four bars could be written like this:
| D | C | G | G |
To give the progression some life, add accents to the 1st beat of each measure. That is, each time you change chords, hit the first downstroke a little harder than your other strums.