Once you get beyond the grip section of the stock, it's the barrel that supports the wood.
With three lugs, there will be longer distances between pins, in other words, longer stretches of unsupported wood. Don't put that first pin too far back from muzzle.
So much of your choice depends on school of rifle(or fowler), whether this is a thin gun or a fattie-boy. If you're not going for an exact school replica, you have leeway for number of pins and placement. Be sure to lay out where the thimbles will go before you commit to the barrel lugs.