It seems to be mostly on the down stroke of the first beat of the bar. What he plays to accompany his singing is
Code:
P P
D D D U D D D U
1 + + 2 + + 3 + + | 1 + + 2 + + 3 + + |
with the down on beat one being heavier than on beats two and three and some times percussive (P).
Played like this it's only the third beat that has the triplet feel.