Light is what makes them grow, so i really don't see the point in 3 days of darkness. I can't see the logic in depriving them of 36 hours of what they love the most...light, when they'll go into bloom within 3-5 days of the switch to 12/12, anyway. Plus, that's about as unnatural as it gets, so it
has to introduce
some level of stress, which isn't worth the risk, IMO, especially when there's no clear benefit in doing it. I
suppose there's a possibility that it will reduce stretch,
a little,(less than an inch, I'm sure) but that's because they're not freakin growing.
WTH are you people thinking?LOL
IMO, if your plants are taking a long time to show preflowers, you had some trouble in veg, whether it be insufficient lighting, nute burn, overwatering, etc....or have some strain that takes a long time to mature. If you started from seed with a 8-10 week strain, and they are growing nice and healthy, you should start seeing preflowers while they are still in veg, after 4-5 weeks(6 weeks max). If not, at least they should have alternating nodes by then, which means they're mature enough to go to 12/12, and will show preflowers within the first week. If not, pick another strain.
The only way i can see that "3 days of darkness" POSSIBLY speeding things up, is if you started from seed and are putting them into bloom at a very young age, 3 weeks or less, and even then...it's iffy.
...and that's my 99 cents.