I think reordering the java specific options to behind the java command solves the problem. In this example, only nogui is a minecraft command line option.
i get an error: [spoiler]
C:\Users\SLICK\spongeserver>“C:\Program Files\Java\jre1.8.0_51\bin\java.exe” -ja
r -Xmx3072MB -Xms3072MB -d64 spongevanilla-1.8.9-4.1.0-BETA-314.jar nogui
Invalid maximum heap size: -Xmx3072MB
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.
C:\Users\SLICK\spongeserver>pause
Press any key to continue . . .
[/spoiler]
Simon gave you the answer but for future reference you just need the first letter, so G for GB or M for MB. All you needed to change was -Xmx3072MB to -Xmx3072M