How difficult could it have been, to predict WWI ... in 1917?
In fact, the first "secret" had nothing to do with WWI; it was supposedly a vision of Hell, with no predictions involved.
The second "secret" "predicted" that the current war would end, but another one would happen in the pontificate of Pope Pius XI, preventable by "the Consecration of Russia to my Immaculate Heart, and the Communion of reparation on the First Saturdays". Otherwise "(Russia) will spread her errors throughout the world, causing wars and persecutions of the Church". There are several problems with this. For one, WWII started in September 1939, but Pius had died 7 months earlier, in February. You'd have thought somebody would have spotted the error, given that this "secret" wasn't published until 1941 (raising the reasonable suspicion that this was no prediction at all, but a retrofit). Also, at the time of the "apparitions" (May-October 1917), Russia, having undergone the February Revolution and under the Provisional Government of Kerensky, was still very much a Xian country, with just as much Mariolatry as any Catholic, West-European land, so calling for its conversion would have been very premature at the time of the "miracles". It would, however, have made much more sense by the time of publication, in 1941, to call for the conversion of Stalinist Russia ... more retrofitting.
You say the third "secret" was never published (to be fair, you did add "to my knowledge"), but, in fact, it was. In 2000, it was published by the Vatican, and it contained no predictions , just a Revelations-style apocalyptic vision and an exhortation to prayer and worship. Cardinal Joseph Ratzinger, the future Pope Benedict XVI, wrote:
Ratzinger said:
The purpose of the vision is not to show a film of an irrevocably fixed future. Its meaning is exactly the opposite: it is meant to mobilize the forces of change in the right direction. Therefore we must totally discount fatalistic explanations of the “secret”, such as, for example, the claim that the would-be assassin of 13 May 1981 was merely an instrument of the divine plan guided by Providence and could not therefore have acted freely, or other similar ideas in circulation. Rather, the vision speaks of dangers and how we might be saved from them.
The concluding part of the 'secret' uses images which Lucia may have seen in devotional books and which draw their inspiration from long-standing intuitions of faith.
What remains was already evident when we began our reflections on the text of the 'secret': the exhortation to prayer as the path of 'salvation for souls' and, likewise, the summons to penance and conversion.