Is the Daycare that he goes to.... a big sized group?
With various ages?
The reason I ask this is: some kids, do not do well in BIG sized groups or groupings. And if the group leaders or providers are not real nurturing... the kids can become "stressed."
At times for young kids, because they do not have instinctive "coping skills"... can become "aggressive" toward others, because they do not know how to manage... frustration and stress.
So, having him in a smaller place with less kids, may help.
Or, perhaps he is just ready for "Preschool" versus a Daycare.
At this age and at 4 years old, kids this age are "ready" for Preschool and often enjoy it. They need more, at this age. And it will also prep him for Kindergarten and he will learn more.
The main thing is, with young kids starting from when they are Toddlers, is to teach the child "coping skills" and HOW TO SAY... what they are feeling. ie: happy, sad, mad, grumpy, hungry, tired, its too loud I don't like it, etc. Because, kids this age NEED to learn, HOW to "express" themselves. It is not a "skill" they automatically know. A parent has to teach them. Because, kids this age get stressed and frustrated many times a day. AND if they are tired or lack sleep or are over-tired... they get even MORE.... frustrated because they lack patience and tolerance... when tired or over tired or over stimulated.
Many kids, this age and on... need to have a time to "deflate." To have a quiet time where they can just be alone or have quiet. It helps them to regroup etc.
From when my kids were 2 years old... I taught them how to say how they feel, even if that meant they are grumpy or irritated. And to tell me and tell me why. That it is okay... I am not just going to scold or punish them for it, because adults too get grumpy. BUT if they know they can tell me, and we practice how to vocalize it.... they can feel better about "managing" their emotions, too.
This is KEY... for a young child. Especially for boys... so they learn how to express themselves and their feelings. If not, they get all pent-up... and explode or, get mean to others.
When my son was only 3 for example, he knew how he felt and would tell me... and knew the differences between him feeling grumpy or irritated or frustrated. And he would tell me... and feel secure in doing so. And that alone, helped him to deflate...
A lot of this is coping skills. Which kids do not know how, yet. Unless they are taught.
You also said that your son does not seem to participate in the majority of group activities. Maybe it is just a matter of him being better with small... groups. And maybe he is introverted? Or maybe the other kids are just too...noisy or overbearing?
What is his personality type?
Introvert?
Shyer?
Extrovert???
It could just be a matter... of the environment.
Also as an aside: when my son is very tired or over tired.. he gets more "hyper" and rough. But we KNOW that about him... so then, he naps or goes to bed earlier. My son if he has lack of sleep or is tired, gets out of whack. Not his usual self.
What triggers... your son to be "aggressive" or hard to handle?
3 is a hard age developmentally, and 4 years old is hard too, or harder.
It is always best, to teach skills to a child... not just punishments. Teaching coping skills and how to communicate or indicate their needs... along WITH... consequences when it is warranted.
For example: some kids get scolded or punished no matter what they do, even if they are not "naughty" but just did something by accident. So then they just think whatever... they will be punished no matter what they do. They have not learned, to know the difference between their behaviors and if it is on purpose or accidental or between bad/good concepts.
So the parent has to teach them... these differences. And then explain why. And then teach the child... WAYS of coping or handling situations.