3 They requested Festus, as a favor to them, to have Paul transferred to Jerusalem, for they were preparing an ambush to kill him along the way.

Read Acts 25:3 Using Other Translations

And desired favour against him, that he would send for him to Jerusalem, laying wait in the way to kill him.
asking as a favor against Paul that he summon him to Jerusalem--because they were planning an ambush to kill him on the way.
They asked Festus as a favor to transfer Paul to Jerusalem (planning to ambush and kill him on the way).