A single plant in a corner looks like a plant in a corner. Three of them, set at different heights and close enough to touch, start to look like a little scene. That is most of the difference between "I own some plants" and "this corner feels cared for", and it costs nothing if you already have the plants.
Here is how to arrange the ones you have, and how to tell when a grouping works.
Before you move anything, measure the strip of wall you want to fill: its width along the wall, and how far you are happy for pots to stick out into the room. Around 40–50 cm of depth is usually enough for a group that stays out of the walkway. Then look at the light. Plants care about the window far more than they care about looks, so a pretty corner that gets almost no daylight is a bad home for a plant that needs it. If that is your only free corner, pick plants that tolerate low light, or let one lamp do the decorating instead.
Two practical checks: keep leaves clear of radiators and heaters, which dry them out, and if you share your home with a cat or dog, look up each plant on a pet-safety list before you put it where they walk. Plenty of common houseplants are not good to chew on.
This is a side view of a corner. The wall on the left is where the plants are lined up, and the strip along the floor is 200 cm wide. Add a few plants, then drag them left and right. The three notes underneath react to what you build.
Your eye reads a row of identical things as a line, and a line is easy to ignore. Put a tall plant, a medium one and a small one together and the eye has a slope to follow, from the top of the tall one down to the little pot at the front. That slope is what makes a corner feel arranged instead of parked.
Stylists often go for groups of three or five, because an odd number looks less like a matched set. Treat that as a habit that works, not a rule that must be obeyed. Two plants of clearly different sizes can look better than five that fight each other.
Put the tallest toward the wall or the corner and let the group step down towards the open side. That way nothing hides the smaller plants, and the tall one frames the whole thing.
Spacing is the second half. If the plants are too far apart, each one looks like a separate decision and you lose the "scene" feeling. If they are packed so tightly that leaves overlap, they shade each other and the air between them stays damp, which plants dislike. Aim for pots close together, with the foliage just meeting at the edges.
Remember that plants grow. A group that looks perfect today may need a hand's width more room in a year, so leave a little slack at the end of the row.
A big plant on its own can be the best thing in the room. A pair is calm and symmetrical. A dense jungle corner, with leaves piled up on every side, is a perfectly good look if that is what makes you happy. And if the light only suits one spot, put the plants where they will stay alive and arrange them there, however awkward the corner.
You do not need new plants. Move what you already have into one place for a week and see whether it works before you spend anything. Prop up a short plant on a stool, a stack of books or an upturned crate to make a middle height. Take a cutting from a friend's plant, or swap one, which is how many of the best corners get filled. Second-hand pots and baskets work as well as new ones, and mixing a few that are close in colour makes the group look chosen.
If you do buy something, measure the corner first, check the pot's width against the space, and check the plant's care label against the light you really have.