What had God already accomplished before Peter preached to Israel in Acts 3?

Before Peter preached in Acts 3, God had already raised up His Son Jesus, accomplished the atoning work, raised Him from the dead, and ascended Him to sit at His right hand per Psalm 110:1. God was ready to return and bless Israel by turning them from their iniquities, if they believed.

Les explains that Peter addressed Israel as the children of the prophets and of the covenant God made with Abraham, saying in Abraham's seed all kindreds of the earth would be blessed. God sent Jesus first to bless them by turning every one from their iniquities. Les teaches that God had in mind Israel was now in a position to accept their King because He had made the atoning work, raised Jesus from the dead, and ascended Him according to Psalm 110:1 to sit at God's right hand until His enemies become His footstool. He was ready to come back if Israel believed. Les notes God made valid offers knowing they would not be accepted, as with Adam and Eve and at Kadesh Barnea.

Psalms 110:1

The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.

Why was Israel in a position to accept their king at the time of Acts 3?

Israel was positioned to accept their king because the atoning work was finished, Jesus had been raised from the dead, and He had ascended to sit at God's right hand according to Psalms 110:1, ready to return. As children of the prophets and covenant, they needed only to believe.