How do you write john 3 16 in greek letters?

There isn't one single, universally accepted way to write John 3:16 in Greek letters, as different translations and fonts may vary slightly. However, a common and accurate representation, using a transliterated form that's readily understandable to those familiar with the Greek alphabet, would be:

Ἰωάννης Γʹ ιϛʹ (for the chapter and verse reference)

For the actual verse itself (John 3:16), a common translation into Greek would be something like:

οὕτως γὰρ ἠγάπησεν ὁ θεὸς τὸν κόσμον, ὥστε τὸν υἱὸν αὐτοῦ τὸν μονογενῆ ἔδωκεν, ἵνα πᾶς ὁ πιστεύων εἰς αὐτὸν μὴ ἀπόληται, ἀλλ᾽ ἔχῃ ζωὴν αἰώνιον.

This transliterates to something like: *houtōs gar ēgapēsen ho theos ton kosmon, hōste ton huion autou ton monogenē ēdōken, hina pas ho pisteuōn eis auton mē apōlētai, all’ echē zōēn aiōnion.*

Keep in mind that different Greek translations will have slight variations in word choice, and the above is just one example. You'll find variations in older and newer translations.