Three groupings all plants go into?

There are no three groupings. If you're looking at the order hierarchically, it would be

Domain: Eukaryotes
Kingdom: Plantae

there is no third division there.

If you're looking at the phylum , there are four
"Bryophytes" a paraphyletic group, as well as many of the sub phylla and groups are not related to each other, but useful since they are all seedless and vascularless plants. The subclassifications are always changing as new molecular data comes out, so it's easiest to use the term bryophytes

Ferns and their allies (seedless, vascular plants) - a polyphyletic group. Also subject to ever changing subclassification

gymnosperms, which may or may not be monophylletic

angiosperms, which contain the incorrect but useful sub-classifications of:
monocots - a monophyletic group
dicots - paraphyletic

This is not counting many of the much smaller organisms which are currently floating in a limbo between the algae and plant kingdoms.

So, I have no idea. My guess is whomever wrote the question either has not paid attention to any classifications past the 1980s or they just don't know about plants. Hope you can wrangle a response out of what I've given you.