This is the logic of F1x DCT. (F8x DCT is slightly different).
1. Whenever you turn on the ignition it goes to Efficient(D1)/Comfort/Comfort/DSC ON as
"default setting" 
2. Whenever you disable M1 and M2 buttons, it goes back to
default setting.
3. There is permanent Drivelogic memory for S mode so if you set the S to S3 it will be remaining in 3 even if you turn the car off and on again unless you have M1 or M2 set to S1 or S2 and you select it. in this case the last Sx will be saved as last setting.
4. There is temporary Drivelogic memory for D mode as long as you do not turn the car off.
So in order to have both S3 and D3 you can do one of these methods:
1. Don't use M1,M2 at all. turn on the car put it in S mode, select S3 and then change it to D and select D3. as long as you don't turn off the car you can switch between S and D and it's always "3"
If you turn off the car, the next time you turn it on it comes with default setting and you have to select the D3 again; however the S3 is already saved as last setting.
2. Set M1 as D3 and your desired setting (throttle, suspension, steering, M view, DSC). Set M2 as S3 and your desired setting (throttle, suspension, steering, M view, DSC). As soon as you turn on the car select M1 and then M2. Then select M1 again as D3 mode. now if you want to change the mode from D to S just change the lever position. you can switch between D3 and S3.... next time you turn on the car again you don't need to select M2 anymore. Just select the M1 to give you D3 since S3 is already saved as default for S mode.
I always use the second method unless on rainy day since I have MDM in both M1 and M2