I have 2 kids. At some time or other, the other one will feel left out. Though I spend time with both of them.
I just say honestly to them "I have 2 kids. Mommy can't be there with you both at the same time. So, I have to take turns. I don't have 2 of me.... " and then they 'realize' and giggle... then are more amendable to it.
I tell them, EACH of them need special time with me... and I "try my best..." and this seems to alleviate the stress of it.
Next, help and teach your 3 year old the names of feelings/emotions and how to express it.. and that he CAN tell you how he feels... no matter what.
I teach my kids that, since they were 2 years old. And this helps. Because then they know they can tell me their feelings/problems/discomforts.. and that "I" WILL hear them and listen. Good or bad/icky feelings... its okay. The home, is where a child should feel comfortable in knowing they can express themselves.
Next, try asking him, what is bothering him. Even if he can't be 'exact' about it... just talk with him.
My kids, when I talk with them, openly, without judgment, they feel better. Because Mommy is RIGHT there, 'hearing' them. And they bond with me that way.
Giving 'time' to a child is important. BUT it is ALSO about building a "Relationship" with them. Seems common sense, but it is not. But it makes a big difference...
Then a child is not just a satellite floating around, being told what to do/punished/scolded/being ordered around by the parent, or not 'heard.'
A child, needs to BOND... with their Parent, at different ages, in different ways. At each age. Thus, building a 'relationship' with your child, is really key and paramount.
Spending time with a child... does not always equate to having/building a 'relationship' with the child. Seeing what their thoughts are/feeling heard/seeing what their interests are and their dreams, having their parent be able to 'see' them for who they are etc., and feeling secure in knowing that they are 'accepted' no matter what.
all the best, just some quick ideas and what I do with my kids,
Susan