I planted (εγω εφυτευσα). First aorist active indicative of old verb φυτευω. This Paul did as Luke tells us in Acts 18:1-18.

Apollos watered (Απολλως εποτισεν). Apollos irrigated the church there as is seen in Acts 18:24-19. Another aorist tense as in verse 1. Corinthians 3:2.

But God gave the increase (αλλα ο θεος ηυξανεν). Imperfect tense here (active indicative) for the continuous blessing of God both on the work of Paul and Apollos, co-labourers with God in God's field (verse 1. Corinthians 3:9). Reports of revivals sometimes give the glory to the evangelist or to both evangelist and pastor. Paul gives it all to God. He and Apollos cooperated as successive pastors.