The whole, "three hammer-blows of fate" thing is such utter nonsense in several ways. Mostly it helped Alma to explain the sixth symphony to herself, and putting the "Kindertotenlieder" into context - for her. Mahler himself stated that the Kinderlieder were not songs about the death of children in a literal sense but about death in general, and our (humans) relationship to death: how we deal with it; how we observe it; how we process it. In other words, it's a metaphor or allegory for death in a more general sense (and hence, it's really about Ruckert). The much maligned Ken Russell movie goes into this detail in a rather fanciful way, but it makes its point.
The worst problem with the 'three hammerblows of fate' concept is that it has the reduced the sixth symphony to being a 'biographical' symphony in the minds of the majority of listeners. There's obviously some biographical element to most works by Mahler, but the sixth symphony is a "tragic" symphony in a more general, 'universal' sense. Mahler even approved of the 'tragic' subtitle. It's far more accurate to think of the sixth symphony as being a sort of Greek tragedy - where the tragic experience is meant to be a cathartic experience - or as something of a warning as to where the world was heading in the most general sense - something that many intellectuals (thinking people) foresaw at that same time. This makes even more sense if you think of symphonies six through eight as being a 'darkness to light' trilogy (with the fifth symphony serving as a sort of model for that trilogy). Whether they were planned as a trilogy or not is really rather irrelevant.
There's another story which makes the biographical element of the sixth symphony quite false. When Mahler premiered the 6th in Essen, he was in a worked-up, almost hysterical state. It wasn't just Alma who had observed this. Yet, when Mahler performed the sixth a year later in Munich, he was rather jovial and focused more on musical matters, particularly the percussion. He may have even left something of a musical clue by having the Liszt piano concerto #1 performed on the same concert. Jonathan Nott goes into great detail on this point in the notes to his recording of M6 (Tudor label).
For anyone who knows the 6th symphony rather well - and especially if you're a score reader - it's clear that Mahler most likely dropped the third hammer-stroke for purely musical reasons. After all, the finale originally had five hammerstrokes! Mahler - allegedly - had spoken about the first hammer-blow being the strongest, and the next two getting progressively weaker. Yet, when Mahler revised the symphony, he offered the optional reinforcement of the second hammer-stroke with cymbals and tam-tam (the bass drum is there in both versions). Most conductors take advantage of that option. Evidence that the Mahler dropped the third stroke for superstitious reasons simply doesn't exist.
I don't think that Alma created the ' three strikes, you're out!' concept as a way to cover her tracks as much as it was simply a biographical tool in helping to explain her life with Mahler to herself. That's just an opinion on my part.