C/C++ :: Editing Vec3 Variables?

I have this function that takes the collision location from an object then creates a new object for particle effects.

This is one of the functions that is called when creating the particle,void Particles_Create(i32 iID, Vec3 &vecPos, i32 ProjectileType, i32 iNumberOfParticles, float fSpread, float fGrowSpeed, float fMaxSpeed, float fMaxSize, float fParticleTime);

I want to be able to change the "Vec3 &vecPos" variable, when I define the variable that goes into the function I've defined it as: "Vec3 vecPos;" if I define it as "Vec3 vecPos(float fvx,float fvy,float fvz);" the variable cant be fed into the function.

I cant change my function because its used elsewhere but I want to be able to change the x,y and z elements inside the Vec3 vecPos variable. How without redefining it as Vec3 vecPos(float fvx,float fvy,float fvz);