Jhn 3:15 That whosoever believeth in him should not perish, but have eternal life.
[...there was a need for the Son of man to be lifted high]...in order that all those believing in him might possess life eternal.
This verse is just a dependent clause that completes the previous verse.
Again, the KJV is misleading because it appears to be talking about the future ("shall have") but the Greek is in the present tense, subjunctive voice. The subjunctive indicates the possibility of something happening in past, present, or future.
The clause about "perishing" does not exist in the best Greek texts today or in recent translations such as NIV. It was an artifact of the KJV from its Textus Receptus source. The phrase was originally from the Latin Vulgate version.
The phrase is directly saying the Christ was raised up in the past so that people now would have the possibilities of eternal life.
ὁ πιστεύων (part sg pres act masc nom) "Believeth" is from pisteuô (pisteuo), which means "to trust, put faith in, or rely on a person", "to believe in someone's words", "to comply", "to feel confident in a thing," and "to entrust in a thing."
αὐτῷ "Him" is from autos (autos), which means "the same," and the reflexive pronouns, "myself", "yourself", "himself", "herself", "itself," or the oblique case of the pronouns, "him", "her," and "it." It also means "one's true self," that is, "the soul" as opposed to the body and "of one's own accord."
ἔχῃ (3rd sg pres subj act) "Shall have" is from echô (echo), which means "to have", "to hold", "to possess", "to keep", "to have charge of", "to maintain", "to hold fast", "to bear", "to keep close", "to keep safe," and "to have means to do."
ζωὴν "Life" is from zôê (zoe), which means "living", "substance", "property", "existence," and, incidentally, "the scum on milk." It has the sense of how we say "make a living" to mean property. Homer used it more to mean the opposite of death.