You have more or less answered your own question in that if the existing btu output from the 3 rads is sufficient to heat the room then, 1 large radiator of the same output will do the same job.
While one large radiator will emit the same amount of heat, it will all come from one area of the room. This wiil inevitably result in hot and cold areas within this large room.
It is advisable to place a radiator under a window to nullify the cold from the window. Hence the radiator arrangement that you have seems well thought out. I would leave well alone, if I were you.
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