My advice: don't panic. I was in a pretty similar position to you with my extension after surgery (hamstring autograft) and my flex wasn't that great either. My PT encouraged me to focus on regaining extension, and told me flex would come in time. I worked super diligently on my extension exercises, and at about six months after surgery I finally started to achieve full extension on a regular basis. It took somewhere between one and two years after surgery for me to finally regain equal extension to my "good" leg, which in reality is about 5 degrees of hyperextension. By the time it happened, I honestly didn't think I would be getting it back. Now both legs are totally equal for extension; the operated side is still a little stiff in the final few degrees of flexion, but it's not something I ever notice day-to-day.
As long as you can achieve full extension, even if it doesn't last, you're on the right track. Keep doing your exercises and your knee should gradually regain the ability to retain increased extension. Three months is relatively early days after an ACLr, and regaining flex and extension is very individual. You can't predict what will happen for you based on a standard recovery. For my part, I regained strength super fast but at the expense of flex and extension which were much slower to return. Other folk who've reported back on this board have found the exact opposite. Each recovery is individual, and needs to be treated as such. Keep working on it, and good luck.