a) What is the third precept of the Church?
The third precept of the Church is: to receive the sacrament of the Eucharist at least during the Easter season.
Aside from this minimum, the Church also encourages us to receive Holy Communion every time we attend Mass. She likewise invites us to attend Mass often, according to our possibilities, even daily if possible.
b) Why should an Altar Server know and live the third precept of the Church?
Through Holy Communion we receive the Body, Blood, Soul, and Divinity of our Lord Jesus Christ. It is the bread from heaven, our spiritual food, in which we receive not only grace, but the source of grace himself.
An Altar Server should encourage his friends to go to Confession before receiving Holy Communion if they have a mortal sin which they have not yet confessed. To receive our Lord unworthily (that is, in the state of mortal sin), is to treat him with grave irreverence; we do not receive any grace, but rather commit another mortal sin, that of sacrilege.