Some might say that the three laws are not really comparable, but the sages constantly warn us against jumping to conclusions as to which laws are more important and which are less.
In any case, theoretically we should not need the promise of reward or the threat of punishment before contemplating a commandment.
The reward or punishment is not an incentive but consequence. It implies, if you keep the commandment, the outcome will be as specified in the Torah.
Is this really provable? If you study the way human history has worked, the proof seems to be there. Look at the three mitzvot:
Honouring your parents? Our destiny largely depends on whether we respect our history (symbolised by our parents) and learn its lessons.
Sending away the mother bird? If we brazenly hurt the feelings of the mother bird we are being cruel, and cruelty of any kind will soon curtail and frustrate the future of our society.
Having correct weights and measures? The stability of a community largely depends on whether we go for balance and justice.