JS Gravity Not Working While Moving RB Controller

Hi I'm Fairly New To Unity And Just Getting To the Hard Stuff
This Has Really Stumped Me
So I Made This Code To Move Using A Rigidbody


if (Input.GetKey("w"))
{
RB.velocity = (transform.forward * speed);
}

This works But when I Jump

if(Input.GetKey(KeyCode.Space)&& isfalling == false){
RB.velocity = transform.up * jumpH;
isfalling = true;
}
Im Not Effected By Gravity (if I press space and W at the same time or W while im in mid air)

so I tried this

//press W and move Forward
if (Input.GetKey("W"))
{
RB.velocity = new Vector3(transform.forward , RB.velocity.y);

but I get the Error
BCE0024: The type 'UnityEngine.Vector3' does not have a visible constructor that matches the argument list '(UnityEngine.Vector3, float)'.

its a FPS
Thx HG