The troglodyte has 2 possible sets of attacks; each time it makes a full attack, it chooses 1 set.
The most important word in the melee section of that stat line is "or". When it full attacks a troglodyte chooses 1 of these 2 sets of attacks to make:
- club +2 (1d6+1), claw -3 (1d4), bite -3 (1d4)
- claw +2 (1d4+1), claw +2 (1d4+1), bite +2 (1d4+1)
The trogolodyte has 2 kinds of attacks: manufactured weapons (a club), and natural weapons (claw & bite). It can choose to hit with its manufactured weapon, which offers a slightly larger damage die but applies a penalty to all its natural weapons that turn, or it can choose to only attack with natural weapons, giving up the club's potentially heavier damage but gaining better attack & damage bonuses with its claws & bite. Another way of thinking about this is that it always attacks with its bite and "off-hand" claws, and merely chooses whether to use a club or claws with its "main" hand; the club applies penalty to the other (option-less) attacks, while the claws do not.
Mechanically speaking, the claw-claw-bite attack set is superior in almost every situation, but a DM might choose to use the club to provide some variety in how different troglodytes attack or simply because the troglodyte is dumb enough to think clubs are better.