Numbers 3 - The tribe of Levi was not counted along with the rest of the tribes. The reason was their ministry primarily dealt with the Tabernacle, its construction, moving, and operation. No other tribe was permitted to handle these important duties. The Levites were the priestly tribe and they conducted the worship services for all of Israel. When God had judged Egypt with the death of their firstborn, God set the firstborn of Israel aside for His service. In this chapter the Lord has Moses substitute all the firstborn of the families of the other tribes to be replaced by the Levites.
Numbers 4 - When the Tabernacle had to be moved it would be the duty of the high priest and his sons to cover all the sacred furniture including the Ark of the Covenant. Only after they were covered could the family of Kohath come and carry these items to the next location. (This is important to remember because David would suffer the death of Uzzah when he loaded the Ark of the Covenant on a wagon). The rest of the structure and the courtyard curtains would be moved on wagons by the families of Merari and Gershon. You can see how the Lord put everyone to work and gave them a sense of responsibility in the His work.