Raymond Brown points out (The Gospel and Epistles of John) the twofold meaning in John 3:14:
“By twofold meaning the word ‘lifted up’ refers both to being lifted up on the cross and being lifted up into heaven. In Jesus’ return to his Father in heaven, the cross is the first step on the ladder of the ascension. Only when Jesus is raised up can the Spirit of which he has spoken to Nicodemus be given.” Raymond Brown
I would add that, just as the cross was the “first step” in Jesus’ ascension, this path ought to be seen as paradigmatic of Christian discipleship. Jesus has gone before us, and where he is there we shall be also (14:3). But it’s also true that where he has gone, there we shall go. We must take up our own cross in following Jesus (Matt. 16:24). But it’s precisely in taking up our cross, in our crucifixion, that we are walking on the path of ascension to the Father. We too must be lifted up, and the cross is our “first step” of elevation.