"They walk according to man." (1 Cor. 3:3). That is his way of saying, "You live like everyone else. Your lives are merely human and are not supernatural. What an interesting rebuke to the way most Christians live.
The evidence of walking naturally is a party spirit where they pick their favorite teachers (This is the third time he's brought it up in as many chapters). They derive pride from who has won them to faith and who taught them.
From God's perspective, each of those planters, waterers and harvesters cooperate with God who is responsible for the increase. Each will receive a reward according to their own effort.
This chapter includes a stiff warning about messing with God's church. If you destroy God's temple, God will destroy you.