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try with

capture >> frame;
capture1 >> frame1;
capture2 >> frame2;
capture3 >> frame3;

frame.copyTo(canvas(Rect(0,0,frame.cols,frame.rows )));
frame1.copyTo(canvas(Rect(frame.cols,0,frame1.cols,frame1.rows )));
frame2.copyTo(canvas(Rect(0,frame.rows,frame2.cols,frame2.rows )));
frame3.copyTo(canvas(Rect(frame2.cols,frame1.rows,frame3.cols,frame3.rows )));