3:8a He who plants and he who waters are one in purpose,

The person who plants the seeds and the person who waters the seeds/plants work with the same goal/aim in mind.
-or-
I began the work among you and Apollos continued the work among you but we are both working to get the same task/thing done, just as the person who plants seeds and the person who waters plants are both trying grow a crop.
-or-
I who first taught you God’s message and Apollos who continued teaching you God’s message both have the same goal/purpose.

3:8b and each will be rewarded according to his own labor.

Both will receive pay/wages in accordance with their work.
-or-
And ⌊God⌋ will pay/reward each worker for the way each has done his work.
-or-

⌊God⌋ will give both/each ⌊of us (excl.) workers⌋ a reward for what we (excl.) do,

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