Well, now there’s a fourth. Here we go!
I think plant civs are unlikely. I’m of the pretty uncontroversial opinion that photosynthesis does not provide enough energy for an active lifestyle, though I’m happy to be proven wrong with math.
I think the biggest barrier is motility. Plants don’t move around much. They don’t need to. There are a few scenarios where it’s conceivable that ambulatory plants could arise, but I feel like they’re unlikely. Remember that, in any scenario, plants will be trying to move from a completely sessile lifestyle to one of any number of niches already occupied by animals, which is a long road.
Carnivorous plants on Earth today are photosynthesizers first and foremost. They get their energy from the sun and only eat to supplement nutrient-poor soil. It’s pretty clear which strategy they prefer since they will opt to make leaves instead of traps if given proper nutrients.
If, however, carnivorous plants found themselves at the mouth of a cave, or a particularly cloudy climate, or the floor of a large rainforest (which also tend to be nutrient-poor), I could maybe imagine that there could be pressure to develop a totally heterotrophic diet. There are, in fact, parasitic jungle plants that don’t photosynthesize, so with a little bit of this and a little bit of that, maybe you could get IRL piranha plants.
Alternatively, perhaps parasitic plants develop a symbiotic relationship with the trees above. They eat the fruit that falls for a little while, move when the topsoil becomes depleted, and take root somewhere new and spread the seeds along the way. Although, this is already done better by animals who, additionally, leave nutrients in the soil instead of taking them.
Seasonal polar predators
Depending on your planet’s axial tilt, the poles will likely have months of uninterrupted sun followed by months of darkness. Assuming that the climate is amenable to warm poles, I can imagine that plants could spend summer building energy reserves and then using those for a brief period during the winter to uproot and hunt hibernating animals and other plants.
Problem is… why would they? Is the soil so poor that it’s worth wasting energy hunting instead of just directing it to reproduction? Might as well just uproot and find a better spot if that’s the case.
Another problem is that such big energy reserves are tempting meals for animals. Those plants are likely to get eaten before they get the chance to eat anything else.