Does Opencv allow you to compute the 3x4 perspective transformation using 6 points?
Yes of course there is. Just look for the computeThreeByFourMatrix() function in the OpenCV library documentation. It is all there
I want to compute a 3x4 matrix transformation, in homogeneous coordinates, that transforms 3d world points to 2d image points. My problem is that in the documentation and tutorials of the function getPerspectiveTransformation the default matrices are either 3x3 for perspective or 2x3 in affine transformations. 
Is there any built in function to compute the 3x4 matrix?
I have read the tutorials, some books on computer vision and I know the matrix that transforms 3d to 2d is 3x4. It is necessary to input 6 points to get this matrix and in the examples I find for opencv they are using 4, so I guess this is a 2d to 2d transformation not what I need. 
cv2.getPerspectiveTransform
cv2.getAffineTransform
I used these functions but it didn't work, I used cv2.circle function to plot what was supposed to be a straigh line on the image and I got curved lines. Obviously I was not using the proper matrix transformation.