querying a completed trilogy of commercial women's fiction?
. . . are there agents looking for this kind of thing even if novels can stand on their own?
Agents are indeed looking for commercial woman's fiction. It's a very hot field.
My advce would be to query only one manuscript -- the first one. I'd suggest not mentioning that you have two more completed manuscripts.
Here's why --
When the agent opens a query and finds you have three completed and unsold manuscripts,
she's going to assume you've been agent hunting for a couple of years. The agent will think she's the hundredth agent you've tried, which is not flattering. And she'll see the manuscript as shopworn.
Once the agent gets excited about your writing, she'll be delighted to know you've got manuscripts waiting in the wings. But maybe tell her about them when she's read the complete ms and wants to sign you.
I don't see any problem with an author's debut book being the first book in a series, especially since this is also a standalone work. Lots of publishers offer debut writers two or three book contracts, so a series might suit the publisher just fine. Your agent will give you better advice on this and know the best way to sell you.