Question: I've heard that publishers are unlikely to consider a single book (novel), or are more likely to consider one if it is the first in a series of three or more. Is that true? Thank you.
If a book sells well, publishers will want to issue sequels. They know those books will also sell well (since demand is already established). This is particularly true in popular genres such as YA, fantasy, or SF.
At the same time, plenty of standalone novels are published as well. In some genres, such as literary fiction, single titles are the norm.
Here's my take on this issue.
Your priority is to write an outstanding first book, because that's always what publishers want.
If a publisher agrees that your book is outstanding and wants to publish it, and if it is the kind of story that lends itself to sequels, the publisher may ask you if you have any ideas for possible sequels. If you have some good ideas ready, that counts in your favour.
If your first book sells well, the publisher may then offer you a multi-book contract to make a complete series.
On the other hand, if the publisher doesn't think your first book is brilliant, they won't care whether you have ideas for sequels. They'll simply pass on it.
Think of it like going on a first date. If the date goes badly, mentioning that you're looking for a long-term relationship will not save the situation. On the other hand, if the date goes great, your desire for a long-term relationship could be a plus.
So what you should do is make that first book great and, if it makes sense, have a few ideas for what a series would look like. Maybe do a brief plot outline for the arc of the series as a whole. But don't go to all the effort of completing additional manuscripts for sequels until you have sold the first book. If the first book doesn't sell, you will have wasted time and effort that could have been spent on a different project.