What are the best three pieces for your money?
It depends. What is your budget? Do you want to go bespoke, made to measure or off the rack?
The most important thing about a suit is a good fit. So, assuming you don't want to spend a couple of thousands for a bespoke suit, you need to find the fit that works best for your body. Pay special attentions to the shoulders, everything else can be fixed by a good tailor. Idealy, you have a knowledgeable sales person who knows what fits you and what adjustments need to be made.
As I said, I mostly buy suitsupply, they offer off the rack and made to measure. Prices start at around 400 dollars for a suit and 600 for a three piece. You won't get better quality workmanship and fabrics at that price point. If you want to spend less my advice would be to rather buy one or two good suits than several cheap ones.
Get a navy suit, a charcoal suit and a light grey suit. Add a couple of Shirts (white and light blue) ties and pocket squares (match the pocket square with your shirt, not with your tie) two pairs of black oxford shoes and you are good to go.