I've been having a knot on my forehead for about 3 years now its not painful but what can cause this? It started off small but now its getting bigger

If. If it is sort of squishy, it could be a lipoma (fatty tumor). You can have your doctor or a dermatologist examine you for an accurate diagnosis.
I. I agree. Hard to tell without a clinical examination but it could be a lipoma or cyst. I would recommend an evaluation by a dermatologist to make sure it is benign. Read more about cysts and lipomas below. I hope that helps.