Someone already answered this question online. Link: http://writersrelief.com/2010/01/11/query-letters-when-not-to-talk-about-multiple-books-including-sequels-a-series-and-other-projects/
Books designed to be part of a series. Certain books are by their very
nature meant to be part of a series—such as some fantasy and mystery
novels. But always keep in mind that before your agent can sell a
second or third book, she or he has to sell a first book. So focus on
book one, and then casually mention that the book could be (or is
being) developed into a series (to make it clear that you’re not going
to put the cart before the horse by pitching second and third books
before the first one has proven that it is viable).
Manuscripts you wrote a long time ago. Many new writers who do not
have writing credits tend to include phrases in their bios such as “I
have written five books.” This leads the agent to wonder: five
published books (if so, why didn’t the writer mention the publisher?),
five self-published books (if so, what is the writer trying to hide?),
or five books that are sitting under a bed somewhere (if so, will the
writer try to pawn off those stale old manuscripts on the agent who
takes that writer on?). If the writer is deliberately vague about the
“five books,” the agent will generally assume the writer is an
amateur. Rather than mention that you’ve written five books that you
were not able to publish traditionally, it may be better not to
mention them at all. Focus on building up your bio with publication
credits in the literary magazine market or by demonstrating a
commitment to the craft.
Self-published books. For more on the question of whether or not to
mention a self-published book in your bio, read Self-Publishing: When
(Not) To Include Your Self-Published Book In Your Cover Or Query
Letter Bio. For more information on how to get an agent for a
self-published book, read After Self-Publishing: How To Find An Agent
And A Publisher For Your Self-Published Book.
Spin-offs. Some writers will indicate that they are already in the
process of spinning their single title book into an alternate
venture—perhaps developing the story of the secondary characters, or
perhaps taking the story to the next level with a prequel. Again, be
sure you’re only pitching one book at a time. If you’ve already
completed your spin-off projects, it may be better to hold off on
mentioning that those projects are already complete until you and the
agent are in serious talks.
Other unrelated books or genres. If you’re writing a romance novel and
you want an agent to work with you on another project (a thriller),
it’s best to focus on one project at a time. The query letter is not
necessarily the place to mention multiple projects. If an agent shows
interest in one of your projects and you feel you can develop a
rapport, then that’s the time to mention your other ventures. But
until that point, try not to overwhelm him or her.
The moral of the story: an agent can only represent one of your books
at a time. Generally, if you are entering a new relationship with an
agent, you don’t want to overwhelm him or her by suggesting he or she
represent previous projects, future projects, or side projects. A good
query letter pitches one book only—and perhaps in certain
circumstances, it may casually allude to other projects (if the writer
is careful in his or her allusions). And if an agent asks if you have
any other projects in the works, then feel free to share. But in
general, play it safe by focusing largely on one book in your pitch.
My opinion is that whether the story is a "novel" or "picture book" really does not matter. A picture book is a story with pictures for children. A novel is a long story for people of any age. They all come in book form, and they all have to be marketed as books.
In the English language, short story, novella, novel seem to be distinct categories. In the Chinese language, there is the 小说, which is divided into three categories by length: 短篇小说 (short, equivalent to short story), 中篇小说 (medium, equivalent to novella), 长篇小说 (long, equivalent to novel). If you are just writing a 小说, then the exact length is irrelevant, unless you want it to be relevant. It can be of any length. The size of the 小说 is measured in Chinese characters.
With that said, a "novel" and a "picture book" are not really on the same plane. A novel seems to refer to the length of a story, while a picture book seems to refer to the physical object the story comes in. It is possible to create a very long picture book based on a novel or a short picture book about the color green.
Assuming that you are attempting to sell 3 children's stories in the form of picture books, and that these 3 stories are independent stories that will be published in quick succession, and that these books are not too much for the literary agent to handle (because a literary agent may find marketing one book too much), you may mention all stories in the query letter. However, I wouldn't be too ambitious with 3 books. You may have 3 books, but how do you know that they will sell? You don't. Given that you haven't published anything in a long time, you may be a bit rusty in this field. So, to play it safe, you may just want to mention only one book in the query letter. If the first picture book becomes a best-seller, then you may mention the other two in the same query letter next time.