The "Etc/GMT+3" time zone ID actually refers to "-03:00", as demonstrated here

etc.scala

import java.time.ZoneId
import java.time.format.DateTimeFormatterBuilder
import java.time.temporal.TemporalQueries

def parseZoneId(input: String): ZoneId =
  new DateTimeFormatterBuilder()
    .appendZoneId
    .toFormatter
    .parse(input)
    .query(TemporalQueries.zoneId)

val plus3 = parseZoneId("+03:00")
// val plus3: java.time.ZoneId = +03:00
plus3.normalized
// val res0: java.time.ZoneId = +03:00

val gmtPlus3 = parseZoneId("GMT+03:00")
// val gmtPlus3: java.time.ZoneId = GMT+03:00
gmtPlus3.normalized
// val res1: java.time.ZoneId = +03:00

val etcGmtPlus3 = parseZoneId("Etc/GMT+3")
// val etcGmtPlus3: java.time.ZoneId = Etc/GMT+3
etcGmtPlus3.normalized
// val res2: java.time.ZoneId = -03:00

/*
"In order to conform with the POSIX style,
those zone names beginning with "Etc/GMT" have their sign reversed from the standard ISO 8601 convention."

https://en.wikipedia.org/wiki/Tz_database#Area
*/