Burning incense is an important ritual in all-major Buddhist traditions. In Buddhism, incense acts as an offering to Buddha, or enlightened ones, as well as a symbol for various aspects of Buddhist belief and an aid to positive thoughts and actions.
In Mahayana Buddhist practice, the 3 incense represents "Precepts incense”, "Concentration Incense" and "Wisdom Incense".
It coincides with Mahayana Buddhist's spiritual practice towards enlightenment, particularly the 3 learning: Precepts， Concentration and Wisdom.
Precepts are laid down to cut off all evils, to prevent bad karma from rising, to purify bad karma, in order for one's Bodhisattva to develop and for good karma to arise.
If a person's mind is full of impediments and bad karma, it will continue to create obstacle to a person's spiritual practice, it will continue to create trouble and sufferings for him. With such obstacles and bad karma in a person, a person will not be able to subdue his own mind, let alone develop concentration.
Therefore, beginners in Buddhism typically focus on practicing precepts to prevent bad karma from rising, to cut off all evil, to allow all good karma to rise up.
And without creating any more bad karma, a bodhisattva pure mind can progress towards the next stage (particular in meditation) such as concentration (at the intermediate level) and the last stage wisdom (at the more advanced level) as the highest level of full enlightenment.
So always offer three incense sticks.