Differential ADC and Common Mode Voltage
I understand that my common mode voltage of my inputs needs to be 1.5V
  as well.


Look at the table you put in your question - it has to be within 50 mV of \$V_{REFCM}\$


  If I had for example, a negative input with 1.5V DC and then a
  positive input of 3V, would that mean that my negative input would
  satisfy my common mode voltage requirement and my positive would not?


\$V_{REFLO}\$ cannot go below \$V_{SSA}\$ or above \$V_{SSA}\$ i.e. it has to equal \$V_{SSA}\$ and typically this is 0 volts. This means you cannot have a negative input.