First time, Jace was heavily affected by anesthesia and was more or less "high" + thinking he was having another wet dream. He then jumps the Duke. Duke should have put Jace back down in bed, but he didn't for some dumb reason. Maybe he didn't want to aggravate Jace's wounds by being forceful? Who knows? Author never gave us any information.
Second time, Duke and Jace are drinking alcohol. Both are more or less buzzed with Jace being almost drunk. Jace then asks to have sex with the Duke another time and we aren't shown that the Duke consented. I think the author is implying that the Duke silently consented because of the sex that happened afterwards. Just...laziness on the author's part.
Third time was rape. Duke offered a compromise, but never gave Jace time to answer properly so no consent. Basically acted like a huge ass. Yaoi logic 101.
Story is honestly just extremely lackluster and more or less just bad at this point.
I under stand the first time jace and the duke had sex , he was drunk am I right ? , but the duke never really stopped him .. second time weren’t they both drunk ? And the third time he forced him to suck his dick and fucked him right in the middle of the hall over him assuming just Bc him and the other guy is buddy buddy with jace that they are dating ... I dunno yaoi has weird logic... there is always rape involved sadly