Um, yah it is his age.
My son recently made 4.... he was actually okay at 2 and 3 years old... but now at 4 years old... he is like that.
You are not alone.
The book "Have A New Kid By Friday" has great tips... it is an easy read and not derogatory nor condescending. The Author is "Leman."
Also the book "Your 3 Year Old" from Amazon. It is a book series on each age juncture and merely describes what each age stage is. Although written awhile ago, it is still very pertinent and informative. An easy read and a "thin" book.
Teach your son, HOW to communicate, the names for his feelings... that he CAN tell you how he feels... but teach him HOW to say it... in a more palatable way.
My son when grumpy or fussy... will actually tell us "I am grumpy... I want to be left alone..." and then, we do. Otherwise, if 'forcing' him to interact, he WILL then, scream and get 'mad.' So... it is also knowing your child's cues... and that they are trying "their best"... to tell you, how they feel/what they want/what is irking them etc.
I tell my kids, its okay to be 'grumpy' and even Mommy gets like that... we are all human. BUT... to "try your best..." to say it in a good way... so we understand.... and we are all a "team"....
Or when you feel at your wits end.... the end of your rope... say "Mommy is going in the other room.... I am upset right now because you are not cooperating.... " then go. It is like a "Mommy time-out..." but to cool down. Not out of punishment.
Also, when your son IS in a tantrum or scream-fest... just do not try and reason with him or lecture him while he is IN that mode. No kid can be listening well at that moment. Just sit down, tell him "When you are done... tell Mommy, I am reading now..." then do it. Do not 'interrupt' his tantrum... a child, WILL deflate on their own. Then AFTER they are done with their tantrum... you talk to him and do whatever you feel is appropriate about that behavior... and telling him, clearly, that it is not going to 'make you' be nice, either. I tell my kids 'being nice takes BOTH people... to cooperate.... I will not give you what you want, when you tantrum..." and I don't.
Also, make SURE he naps. Tired and over-tired or hungry kids... melt-down, more.
Whenever a child is at the cusp of an age change... they get tweaked and more 'difficult.' It is growing pains.... of emotions and cognition.
A 3 year old... if this helps... does NOT have their 'emotions' fully developed nor is it even fully 'mastered' yet... they still need help.... COPING with their frustrations and what not. So, teach him coping-skills.... and HOW to handle, difficulties and upsets.
I don't bribe or reward my kids, just to get them to behave. Otherwise they just expect, the "treat." I praise them.... instead.... and not expect them to be "perfect" (which is unattainable for a kid), but just that they "Try their best...."
all the best,
Susan