Yes, Okra hate being transplanted. They're best started, wherever you leave them for the season. Any ONE of the okra you planted in the pot of three, EACH needs about 3-4X that much soil to come close to full potential.
The thing about okra is, they go through a lot of water (they readily perspire, with the plants even damp to the touch if it's not windy), but you don't want the roots soggy either, so a greater volume of soil to minimize soil moisture fluctuations helps a lot.
I don't understand why you'd put three in a pot too small for one instead of each isolated to its own pot and then, fertilizer far better isolated too.
I just direct sew okra, right into the native soil in the ground. I've tried 6 gallon containers per each 1 plant and it stunts them. If I were to estimate, the point of diminishing returns would be about 10 gallons, but of course this depends on the length of the growing season too. Mine isn't particularly long, about mid-may it's warm enough, till freezing weather kills them in early november, 6 mos. They're usually about 9-10 ft tall at that point. They just get standard 10-10-10 fertilizer, whichever brand of 40-50lbs bag of granular is cheapest at the time. No mixing fertilizer into water, just throw out some fertilizer every few weeks, and some at first right before I till up the soil to sew the seeds.
If the native soil is poor enough, I till more material into it. I make a line with a seed every ~3 inches then cull away those too close to each other that aren't growing as fast, so I end up with about 1 plant per ~foot (minimum) in the row.
Ultimately I feel like my limit on yield is the length of growing season and amount of area for solar exposure (use a larger ground surface area, longer row or multiple rows), not a special soil or fertilizer. If each plant got to be 30% larger, they'd just crowd each other more competing for sun. Plus... I'm happy with 9-10ft height, means I can bend the tops of the plants down just enough to harvest without them breaking from excessive flex angle, instead of plants taller then having to get a ladder out to harvest, 1 to 2 times a day near the end of the season.