Some believe John is referring to the whole world, as in Jesus died for everyone in the world and therefore the whole world, every single person in the world will be saved (universalism).
Others do not believe all will be saved but rather Jesus died for all in spite of their unbelief, so they believe Jesus did not die for anyone in particular.
Who John is referring to when he uses the word world is a question dealing with the extent or limitation of the Atonement (satisfaction or restitution for a wrong, injury or offense).
Christ work was sufficient for all however it is only effective for some, those who the Holy Spirit of God draws, opening the heart to believe the gospel like Linda in Acts (16:14), the elect those that He gives eternal life to, who will never perish and no one is able to snatch out of His hand (John 10:28).
Christ is revealed in scripture as having satisfied the wrath and justice of God for those who believe. This is evident by the preceding verses of John chapter 3 mainly v.18 “Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God” (ESV)
John cannot be referring to universal redemption rather he is saying that all Christians, current and future, Jews and gentiles will be saved by the work of Jesus. In other words this work of Christ is for all His elect, Christians of all ages.
John in Chapter 3 informs his readers more then once that some will perish (non-believers) therefore it cannot mean all in the world will believe and be saved from the wrath/justice of God.
John is saying, there is one Savior, the Lord Jesus Christ for all Jews and gentiles versus there being a Savior only for the Jews who were reading and hearing this message. So Jesus is not exclusively for the Jews.
This served to tell all especially Jews that gentiles had the same Savior, “For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility” (Ephesians 2:14)(ESV)
Paul uses the word world in a similar manner when he writes, “that is, in Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting to us the message of reconciliation” (2 Corinthians 5:19)(ESV)
Paul is not referring to universal salvation rather he is saying we have one Reconciler, The Lord Jesus Christ. This truth is better understood when the whole thought is considered in its context for the next verse reads, “Therefore, we are ambassadors for Christ, God making his appeal through us. We implore you on behalf of Christ, be reconciled to God” (v.20)
Why is the Apostle imploring men to believe if they are going to be saved anyway?
This truth can even be seen in the Great Commission to proclaim the gospel, “whoever believes and is baptized will be saved, but whoever does not believe will be condemned” (Mark 16:16)(ESV)
The Evangelist is told up front that not all will believe, therefore they will be condemned.
My God bless all the readers and doers of His Holy Word!