He is only going to be 3 years old.
They do NOT know themselves yet.
At this age and even older.... their "emotions" are NOT even fully developed yet... NOR do they even understand every single nuance of an emotion nor the 'names' for it, nor even know how they feel. Not even Adults... know that sometimes.
So... keep analysis and expectations... of him/his emotions... age appropriate.
Next: for "Sharing".
I think... to me, a child does not have to carte blanche SHARE everything. For my kids.... and when I had my 2nd child... I explained to my daughter that she does not have to "worry"...that her things are her things. I Understand... they are special to her. She does NOT have to share everything w/her little brother. SHE can decide, what to share or not. Its okay. Frankly, I would not want to share my closet of things/clothes with my siblings... NOR have them at will go in my "personal" space or closet and just grab whatever they want. A Toddler... needs to know... they have Boundaries too. THEIR things. A Toddler...worries about these things... that they ALWAYS HAVE TO share their things. But they do not have to. Teach him that... and HOW to say it... to you. And show him that you understand and have respect for 'his' things too. After all... as your baby gets older... he will have to learn... that he CANNOT just take his Big Brother's things all the time either... just because he is the youngest one. It works both ways. Respect for other's things...
For my Eldest child... I always told her "YOU are my first baby... and always will be...." and it is true. But it makes her feel special. I always... talked with her... AND let her express herself/her feelings good or bad... and she felt 'safe' in doing so... because I never 'scold' or punish... for her feelings... but taught her about... her emotions... and the names for it... since she was 2 years old....
Yes, a child THIS young... does not know how the world works yet... nor their frustrations. Normal development. He KNOWS you love him... he is just at the "contrary" age phase.
The books "Your 2 Year Old", or "Your 3 Year Old" or "Your 4 Year Old" is a good series... on each of the age junctures of children. It merely describes... what each age is like. It is an easy read. Although written years ago... it is still... quite pertinent....
Your child is young... he is developing... they are not 'big kids' yet. Not even emotions or impulse-control is fully developed yet. They are a rock collecting moss....
all the best,
Susan