What about other adcs? Using vayne, the designated tank buster, as your rationale as to why the item is bad seems a bit odd.
That and I doubt you could only take 3 more autos. Let's gave a balanced 3 item tank build with spirit visage, sunfire cape, and gargoyles. Your tank now has 140 bonus armor, 135 bonus MR, and 875 bonus health. At this point your physical damage reduction is about 70%.
Let's assume level 14 as the 3 item point for both champs.
Vayne with her standard first 3 items has +65 ad and a bork passive. Guinsoos will not be stacked since you're using stoneplate at the start of the fight to maximize its value. Her total AD is 158 at this point.
Maokai will have 1895 base health + 875 bonus hp for a total of 2770 hp.
So, you activate the item and gain 2000 hp (doubling your current hp, assuming you took damage during the engage).
To validate your claim of 3 autos being all it takes to kill you, here is the equation. I'll even simplify the bork damage and increase it in your favor (for calculation simplicity).
2*(158+(.084000)).3 + (158 + .7(158)+(.084000)).3 + .14*4000.
Those 3 autos just did about 1023. Now, the actual damage you did will vary a bit based on your keystone rune and some other factors like crit. However, those factors arent getting you the extra 1k damage you need to claim 3 more autos will kill a tank popping stoneplate.