I've got a very slight buzz coming from the D string on frets 1 to 3. it's only very slight but is there any way on fixing this or should I just leave it?
If you're comfortable with your action and you can't hear it out of your amp, I say leave it. If it's bothering you too much, just raise the action on your bass strings slightly.
^That's because the bridge will usually only raise the action from the 10th fret up. Is it HORRIBLE buzz? If it's only slight and you can't hear it through the clean channel of your amp, I'd say just leave it.
It's when I'm hitting the string harder than usual. It's not thrashing away on it but it's maybe a bit harder.

It's not horrible but just a bit irritating. I had it before on the bottom e and i coudlnt really hear it through the amp.