Not letting you know, or not letting you have a say in it, is discourteous in my book.

'just us' lunch dates with most friends.

Seriously, if it happens, let the friend know you'd like a heads up first in the future because you thought it was just gonna be a twosome and you have had a crush on said friend for years and friend keeps **** blocking with a third wheel, not a real threesome.

...this sort of thing happens quite often in business meetings.

Usually that's a good thing though.

I have something worse than a lunch though. When I was single, my (then) best Friend and his wife brought her best friend to a two week trip to Aspen without telling me a) that the woman was coming, and (b) that the woman planned to sleep in the "extra" bed in my room in the Condo whose price we split. (The woman couldn't "afford" to pay for the trip, and they just assumed I wouldn't mind.)

It was an uncomfortable two weeks.

The next year I came with my future wife - being sure to tell them way in advance - and they showed up with their two year old, and then they complained about our, um, lovemaking since their son was only two.

Our friendship went downhill from there.

...definitely ask. Or at the very least, inform you if plans are changed.