You'll find many contrasting viewpoints on when the earliest you can effectively flower is. Once you see pre-flowers, once you have sets of 5 leaves, once the nodes become unbalanced, or some count of total nodes are all methods i have read on when to flower. It really depends on your specific grow area and lighting and doing what best suits you. I wouldnt flower a seed at anything less than 5 nodes, however my plants are clones. because clones have the effective age of their mother, they can be flowered whenever you see fit.
3 months of beg is alot of veg time, no matter what type of lighting you have. i mean honestly, that thing is going to be a giant when it flowers. how big is it now? your going to have to do some serious pruning, but dont forget you cannot cut off more than 1/2 of the leaves or the plant will die (general guideline, im sure theres exception cases).