Is it slipping as you pedal, or from one gear to the other, if its as you pedal it could be the teeth on the cassette maybe worn, if its gear to gear what
@Ajax Bay said, though could be other things.
If its worn teeth, I say this as you say its slipping in 2-3 which could have been the previous riders favourite gears (get the idea, most used), it will mean a new cassette and chain. 9sp sora won't be expensive, you could take the opportunity to re-gear the bike if you want if you are finding hills to difficult or easy.