Question Need Help

I am trying to move a player along the three, world axis, so if I move in one direction it takes longer to change direction, or I can specifically add or remove speed to a specific direction.

My problem however is that I have a vector called Player, and wrote this code to control it.

Code (CSharp):

1. Movement.x = XVelocity;
2.       Movement.y = YVelocity;
3.       Movement.z = ZVelcoity;
4.  
5.       Player.Move(Movement * Time.deltaTime);

so if i make Xvelocity 10, the character will move +10 along the X axis a set amount of times a second.
if i then want to move back, and only add -5 to the XVelocity then it will mean that I'm still moving at +5 on the x-axis.
The problem occurs because I want to make it when i press "W' for example, I add a number like 10 to the direction I'm going moving me forward, relative to the direction I'm looking, but if I'm looking between to axis, then I can only move them along the world axis meaning i cant move forward.

Anyone please help/

 

Is that only when you "W" key pressed?