None of the mentioned alternative, that's how you should say:
I hope you slept well last night
When using last night, which is a period of time, no preposition is necessary, at/on/in, none of them will be used before last night, and it also works for others periods of time: last month, last day, last week..
I hope you did well in the job interview last week
I hope he gave you the present I made for you last month
I hope you looked after my son as I was out last hour
As you can see, no preposition is used before a period of time that includes the word "last".
And if you are talking this night, it doesn't take preposition too, see: I hope you slept well this night. = This night = today's night
You are only allowed to use prepositions in that sentence if you are talking about a night, not a specific one, from which a period of time hasn't been mentioned, see: I hope you sleep good at night/morning/evening.
The night in here is very cold, I hope you sleep well at night.
The morning here is very hot, I hope you can sleep well at morning.
John and Peter can't sleep at night because the night is cold, I hope they get used to this.
Thanks for Araucaria for pointing out some mistakes.