This actually happened when I was in nether and I got completely surprised, astonished, flabbergasted, amazed, whatever term you prefer. If you wonder which weapon I was using, I used spirit sword (untradeable halloween item, can be purchased with real money).
Spirit sword has 2% chance for critical hit. This means it has 1/50 chance to happen, so it is quite rare.
To calculate probability for 3 critical hits in row, we will need the next formula: 0.02 x 0.02 x 0.02 = 0.000008%. So it basically has a chance of 1 in 8*10^6 or 8 000 000. Believe it or not.
This probability can be compared to finding 2 legendary items in the same black tower. Insane.
I know what I saw and I’m not even lying. I’ve never seen anything like this. Ever.
And if you are asking for proof of what happened: sorry, there is no proof.
Sorry if I miscalculated something and caused anger, please correct me if I’m wrong.
Have you ever got 2 critical hits in row? Or maybe 3 or even 4?