You're the one rewriting history. 3-5-2 with Van Gaal didn't work, that's why he changed back so quickly. The defenders struggled to work with each other and our wingbacks weren't fit enough to play the system. Plus Van Persie and Falcao kept getting in each other's zones, because they were too similar. The current 3-5-2 is very different. But if you're looking for the link between our current (very sporadic I might add) use of 3-5-2 and Conte's arrival, allow me to illuminate you.
1. Conte arrives, and after trying a couple of systems that didn't work, changed to 3-4-3, thereby finding a system that other teams struggled to cope with. It meant Hazard didn't have defensive responsibilities, and no one could outrun them in midfield because Kante did the work of two players.
2. Other managers, seeing the gross success Conte was having with the 3-4-3, studied his formation and looked for ways to respond to it. Spurs and Arsenal were two notable teams who changed their system specifically to face Chelsea, and then carried on afterwards. Meanwhile, United changed the system just to play against Chelsea (last season at least). That 3-5-2 was very different; the CB's understood when to go and when to stay, our attackers were young, quick players able to stretch a back 3 more than injury-prone players approaching 30 ever would, and Pogba had cover because we used 3 central midfielders, rather than two and a #10. This is distinctly different from LVG's brief use of a similar formation (but clearly a different system).
3. We have since occasionally used 3-5-2 to respond to other teams playing 3 at the back (as playing a lone striker against a back 3 leaves you very isolated). This has nothing to do with LVG's use of the system, but rather other team's use of 3 at the back to counter Chelsea (as I've already outlined).
You're being stubborn and disingenuous to continue to suggest that current use of the 3-5-2 by us or anyone else is down to Van Gaal. This is obvious to everyone else, and to continue to argue otherwise is to do so for the sake of it. Spare the rest of your dignity before the caf is forced to take it from you.