According to Rashi, this third aliyah took place on the day the Mishkan was erected. This is because G-d's presence was now manifest in the Mishkan and so G-d gave rules (mitzvahs) to guard the Mishkan's sanctity.
In this aliyah G-d tells Moshe to instruct the Jewish people that whenever someone becomes tamei (ritually impure), they are to be sent out of the camp. Rashi tells us that there were three camps: first, the Mishkan itself, second, the camp of the Levites around the Mishkan, and third, the camp of the whole Jewish people (Machne Israel).
The three types of impurity referred to here are: tzoraas, the spiritual disease of the skin; the zav, one with a male discharge; and finally, one who is tamei mais, who is tamei from touching a dead body. Someone who is tamei mais is barred from the first camp, the Mishkan. Someone who is a zav is barred from both the Mishkan and the camp of the Levites. And someone who has tzoraas is sent out of all three camps.
The third aliyah then mentions how one who sins against his fellow man must restore the principal plus one-fifth.