Error Code (121014): Net HEX2, HEX1, HEX0 cannot be assigned to more than one value. If someone can help that would be greatly appreciated.

On these lines:

bcd_seven_seg_behavioral u0(SW[3:0],HEX0);

bcd_seven_seg_behavioral u1(SW[7:4],HEX1);

bcd_seven_seg_behavioral u2(SW[9:8],HEX2);

the second arg HEXn is an OUTPUT of this function.

And nowhere do you assign a value to 'hexN' before using it.

I think the above lines want to be:

bcd_seven_seg_behavioral u0(SW[3:0],hex0);

bcd_seven_seg_behavioral u1(SW[7:4],hex1);

bcd_seven_seg_behavioral u2(SW[9:8],hex2);

otherwise you are assigning a value to HEXn via the assign statements, and also the bcd_seven_seg_behavioral module output.

Note verilog is a case sensitive language.