Ashish Mishra,
Yes, Your understanding is correct.
For your 3rd scenario(accessing DDR via EDMA3 and DSP), You have to look in your code or design part.
TI provides the EDMA test code in ompl138 Startware.
For reference OMAPL138_StarterWare_1_10_03_03\examples\lcdkOMAPL138\edma.
Kindly understand the sample code.
The data corruption is not because of EDMA, how you are accessing the DDR via EDMA and DSP(same timing).