If you do not want to axis to move do not put it in that line of code.
X100Y25--this will move your X100 and Y250 at the same time
Z50--This will move to the 50 once the previous block is complete
Your example of the program
X100Y250Z0-- this moves X100,Y250,Z0
X0Y0Z50--now you will move X,Y back to 0 and Z 50 positive.
I am not 100% on what you are trying to do.