If he takes this to court the court will want to see attempts have been made at mediation before they do anything. They will probably expect to see contact increased as your dd get's older, but slowly and not usually in big jumps. Three overnights every other week is basically the 'standard' every other weekend and half the hols that is usually dished out once a child starts school. That is still another 2 years away...
You appear to be being reasonable. So don't worry, as long as you are the one being reasonable and he is the unreasonable one he won't get far in court. He will get told to act reasonably should he go and see a sol or take it to court and that what you are offering is fair.
Ask him why he wants this sudden change. Ask him to think about dd and what is in dd's best interests and to consider how this might impact on her. Ask him to go away and think about it and to make sure he is asking for this because it is best for her and not because it is better/easier/more convinient for him.
Tell him you have no problems with increasing contact, and as she gets older the overnights will increase. But make it clear that you believe that this is too big a change at her age and that increasing by one day and seeing how that goes is a start.
Should he go to court it will take several months to get anywhere with this. By which time he could have simply accepted the additional overnight with the understanding it will move forward in time and he won't have the stress, worry, bills, bad feeling between you and everything else that comes with court. Maybe explain that to him and he'll see sense. On the other hand he might take it as a threat and set out to 'prove' you wrong. Only you can know your ex. Is he generally a reasonable person?
Oh and one last thing. I think it's unhelpful to say that your dd's hyper behaviour is down to all the sweets he gives her. Yes she might well get treats etc from a dad she sees once a week. On teh otehr hand she may not. She might also get treats from her grandparents, aunties, your neighbours and friends. You might even give her a treat yourself from time to time.. Do you get as upset over that? Any change to routine can lead to a tired/excited child and we all know that tired and excited children can be more challanging. It's not anyone's 'fault' that this is the situation you are in, you just have to deal with it. Things will probably settle more as time with her dad increases and a routine between you all develops. He won't feel so much like the 'fun time, treat parent', and maybe you won't see him as that either...
That's just one of my pet peeves, not criticising, just trying to help you to see another point of view.
As far as the additional overnights go I think you are being very reasonable and he is mad if he doesn't see it too and decides to take it to court. He really won't get much further any quicker.
best wishes
Gilly