1) Had I known there were going to be this many guests, I would've ordered more food. Correct. Fairly formal. 2) If I had known there were going to be this many guests, I would've ordered more food. Correct. Standard. 3) If I would've known there were going to be this many guests, I would've ordered more food. Incorrect. However, you will hear native English speakers say this. Don't copy it. NB Note I've corrected 'these' to 'this' throughout.