From symbol rate to sample rate fractional ratio
The symbol rate is independent of the sampling rate. You can sample the symbols at any desired rate that is greater than twice the bandwidth of the symbol, where higher rates would simplify the required analog filtering after the DAC (so higher sampling is better in this regard and the initial ratios the OP is using are reasonable).

The bandwidth of each symbol is driven by the pulse shaping that may be used. With typical pulse shaping such as Raised Cosine, two samples per symbol would be a reasonable minimum sampling rate to use but would also require tighter analog filtering. 

To see this, consider each “symbol” as it would appear as an analog waveform and observe how you can sample that any any rate as long as you observe Nyquist’s criteria to sample greater than twice the highest bandwidth of interest in the signal. Going from digital to analog with a DAC is this same process in reverse, in that how many samples do you need to properly reconstruct the desired waveform.

Assuming this design starts with symbols sampled at one sample per symbol at a 3 MHz symbol rate, typically this would be interpolated (upsampled by inserting zeroes and then filtered) where the filter would be the pulse shaping filter defined by the waveform transmitter specifications (often a root-raised cosine filter but must be defined).  This upsampling could be matched to the transmitter sampling DAC frequency using polyphase resampling or done in stages.  If in stages resampling to only 4 MBps would not be sufficient for pulse shaping given the explanation above, but it would be reasonable to resample to 10 MHz for pulse sampling and followed by 2x interpolation for the final DAC sampling.