This is a 3D question.
Let the typical NS and WE map be the xy plane
With z the height.
Then:
x = - 300
y = 500
z = -500
D = sqrt(x^2 + y^2 + z^2)
D = sqrt[ (-300)^2 + (500)^2 + (-500)^2 ]
D = sqrt[ 590000 ]
D = 768.11
Yes, the target is within 1000 meters.