Eirka K. Had some great suggestions.
With a 3 year old... its a hard age.
For a boy, you might nurture his ability to communicate and express himself.... to KNOW himself, and that he can express his feelings... etc. Whether for good or bad, it is important for a child to 'know' that they can express themselves, feel validated, and then talk about it. It will in the long run, foster a 'foundation' in a child on how to communicate... and verbally say what they want/don't want etc. Then for the parent, it is wonderful to see our child be articulate in terms of their verbal skills AND in their blossoming emotions... and how they can tell us about it. Which then translates into, our child being 'able' to tell us about their day, their problems, their happy moments, anything under the sun, and that we are their soft place to fall no matter what.
From this age, it greatly benefits a child to learn 'how' to express themselves constructively... even if they want to grumble about something. And knowing that they are 'heard.' For my daughter and son, this helps deflate things rather well.
If it is something I disagree with, that they are telling me... I explain, let them respond, then there is an outcome... they learn that things then go full circle... there is a "conclusion" to things and how they feel... or that there is a "productive" way to go about it. It is a SKILL and "problem solving" that a child then learns... and it will serve them will in the future.
This helps them to realize how to talk/speak/communicate, instead of "battling" for it... or that it is a battle and that it is "futile" for them. For kids, at this age... it is typically always the kid being at the short end of the leash or at the short end of the stick. AND, then no one really validates their feelings nor helps them to understand it. Feelings at this age is STILL developing. It is not fully evolved yet. Thus, they act out.
ie: "I know you are upset... adults get frustrated too. Let's see how we can problem solve that. How can we make it better?.. but I"m glad you told me how you feel, otherwise Mommy wouldn't understand you..." This is pretty much what I do with my kids. It lays a manner in which they will then learn... about "managing" situations. THIS then enables a child to understand "boundaries" and that it is not always negative, nor denying them, nor banishing them from things. But that there is a CHOICE... always, in how to handle things. It is not always black and white. AND, if it is actually MY fault that I triggered my child into a BAD temper, I always recognize that and apologize to them... for my Daughter, this always matters to her, in her heart. And it softens her reactions. Sometimes kids just feel miffed and always at the end of the stick for no reason and being told 'no' all the time.
Next and MOST importantly, your son is now feeling he is "2nd fiddle" to his baby brother born ONLY 3 months ago. THIS is also normal for a little child to feel. In my case, I had my son when my daughter was also about 3.75 years old. For me and my Daughter's Preschool Teacher at the time, after I had my son, I told them that my Daughter is adjusting to a new baby. AND they worked WITH her, not against her, because they UNDERSTOOD that my daughter JUST had become an 'eldest' child and had a baby, too, not only Mommy. So TOGETHER, we worked harder with our eldest child... to help her integrate, to feel wanted, to feel like she still matters, that she is STILL IMPORTANT and that her life is not just about being told rules and 'no' and being controlled all the time. That she is only human... and that she is having difficulty TOO, to having a new baby in the family, in the house, taking up Mommy's time, taking a lot of attention, and pretty much changing HER life. So... a child needs LOTS of time to adjust to a new sibling. It can even take a couple of years.
In the meantime, they need to be 'taught' about caring, about being loved together, that they are still loved, about how baby loves him TOO, about how they are now the 2 Muskateers... to engender commonality between the 2 siblings not differences, and Teamwork. My daughter, loved her brother... we also taught her all about baby development, age appropriately, in order to help her understand... and it made a real big difference. She then took pride in having a baby brother etc. Not that baby was a "competition" for attention. Which some children feel.
A great book is "Your 3 Year Old" or, "Your Four Year Old" which you can get from www.amazon.com
Just some ideas, all the best,