I am worried about this bump on my forehead. The more weight I gain the bigger it gets. I've had it for 3 years now. (Photo)

You would need to be examined to try and determine what the lump on your forehead is.  The most likely causes would be either a lipoma (if it is soft) or bone (if it is hard).  There are surgical treatment options for either cause.  I would consult with a facial plastic surgeon or plastic surgeon to be evaluated.  

Without an exam to feel the mass, it is impossible to make an accurate
diagnosis, but based on your description it is most likely a lipoma (a benign fatty tumor). The treatment is excision. The incision
could be made in a natural forehead crease for optimal scar camouflage
and would likely be smaller than the mass itself. Another option is to
make an incision at the hairline and excise the mass from up there. Be
sure you see a facial plastic surgeon or plastic surgeon. I wish you
well!