The key word in 1 Corinthians 3 is 'reward' (vv. 8 and 14), not purification — establishing the passage's actual theme

Verse 8 ('each one will receive his own reward according to his own labor') and verse 14 ('he will receive a reward') are the bookends of the passage. 'Reward' is mentioned again in 1 Corinthians 4, making it a consistent section-level theme. The word 'purify' or the concept of purification does not appear anywhere in 1 Corinthians 3. The transformative work inside a person is not in view at all.