To be fair 3-4-3 is very hard to pull and very fragile system if not all players are on the same wavelength. Cruyff's philosophy was making the pitch small, press and also make those triangles all over the pitch where a more proactive and energetic approach all over the pitch would mean working a lot on and off the ball with high energy.
- His Barca team depended on the build up from deep where he had not only Koeman but also Guardiola who started the possession from the back whereas Koeman was usually the one distributing long balls to the wing.
- as a LCM in that 3-4-3 and without a proper LB to support if the team faced a proper wing back or a winger he could prove to be a liability given the team mechanics(playing 3 at the back) and also him lacking a bit of pace and dynamism when covering that side.
A good explanation of the 3-4-3 by Cruyff himself:
Notice the #4 role (Haan here) and how Cruyff explains that he shouldn't be allowed to leave the center circle at all. You'd need a highly dynamic B2B on that side, instead of van Hanegem when playing in a 3-4-3 and without a proper LB support, otherwise a winger/wing back could easily stretch him and Montero.
You've named some of my favourite aspects of this formation and Cruijff's style.
What you said about Van Hanegem as a liability when facing sides with a winger or wingback is something I find really interesting about this formation both under Cruijff and Van Gaal. For example, if you watch the full match from which I selected the clips for that video I (tried to) linked in my previous comment, Seedorf played in that left half position. Compared to Cruijff's system, the team as a whole fell back and compacted more than Cruijff's version. Under Van Gaal this meant Rijkaard dropped back to cb, with essentially a midfield block of the two halfs ahead of him. As you say though, this type of compacting and stretching vertically, and as Cruijff would argue, means you're defending and having to work over much larger areas of the pitch.
Alternatively, and what Cruijff discusses is rather than the back of the diamond falling back as a centreback, the halves were expected to track opposition runs on the wing.
Offensively though, referring back to that Ajax match, Van Gaal's halves did something similar but reversed in that they came to collect the ball from the 3 centrebacks at wing-back areas and from there either carried the ball forward and towards the centre, or looked for the pass to the central "kapstok" (player at tip of diamond who plays as a target with back to the goal: Bakero as a midfielder under Cruijff, Kanu as a striker in Van Gaal's system).
This is different to Cruijff's idea of the outer centrebacks as ex-wingers capable of who make wide runs forward in possession. As I said in my previous comment, this contrasts with Van Gaal's use of ball playing centrebacks who pass from wide areas around the halfway line,
I also thought Cruijff's explanation that playing long passers centrally allowed them to reach either wing over the same distance was great, and for that reason I didn't understand Pochettino's decision to play Sanchez as the central centreback between Vertonghen and Alderweireld in the first half of the season. Alderweireld's crosspass is the strongest of the three, and Vertonghen and Sanchez' strengths lie in their ability to carry the ball forward. (besides, Sanchez' strength lies in his aggressive forward defending, whereas he's suspect in his ability to defend towards his goal, which was his main duty as the covering cb in that system)
Also, I'd like to use this thread to mention my severe ongoing annoyance that there seem to be zero full match recordings of Ajax managed by Cruijff.