S.H.
They get "fears" at this age. Some kids start from 2 years old. And when kids are older as well, they get "fears" as well... both my kids go through that with their respective ages.
Just kinda validate him on it... sometimes when we try to talk them out of it, it can just make it worse... but if they feel like someone really understands them... it can bring them solace.
The fears can come and go, be transient, or throughout toddlerhood... ie: fear about the dark or night-time noises etc.
And yes, sometimes if we get desensitized to it, then when something "really" bothers them, we won't know it.... and they know when we don't believe them.
I don't believe it is gratuitous... he is just still learning as well... about what different words mean...ie: vocabulary. AND.. their emotions are not fully developed yet either... a 3 year ls no expert on "emotions" or coping. So, they need us to help them discern it all to them and to clarify or explain or help them feel reassured.
For instance: if he said he's scared... ask him 'what does the word scared mean"? For my son, I ask him what a word means that he says... so that "I" can be sure what HE means. Sometimes, (afterall they are only 3 years old) they may not mean that exact word. But something else.
For example: when I correct my son on something (not scolding but just explaining something) he will tell me emphatically "Don't BLAME me Mommy.... me good boy...." So I just asked him "Honey, what does 'blame" mean????" and he said "me bad boy.... " And so "I" had to correct "my" reaction to him TOO... because for him 'correcting him' on something, TO HIM, was in his mind being "blamed." So I explain to him that I LOVE him... that Mommy is just explaining something to him... he did not "do" anything wrong.. I am just teaching him something.
So, get clear on what exactly your son 'means.' Don't immediately assume he is just saying empty words or feelings. Explore with him, his words, how he "thinks" it means and what he is really feeling. AND yes, teach him that it is "okay" to have fears. Nothing wrong with that. Kids WILL go through that no matter what... and it will not always make sense to us adults. Kids minds and imaginations are very active...
So the "fear" stage... is 3-pronged: the child having "fears" which IS developmental based... AND the child's understanding of words/vocabulary and its concepts.... often, they are simply not articulate about word "definitions" like adults are. So teach him. AND then their "emotions" and their feelings and how they interpret and understand things...
Oh and another thing: try not to convey that "boys" have to be "strong" and only "babies" get scared of things. This will make them think that they can't express themselves or cry or even get scared of anything. At least... your boy will admit/tell you how he feels. BRAVO! Nurture that.... fears and all.
All the best,
Susan