Go up to the attic one time and install three different wattage lamps taking note of which lamp is in which socket, go back down and use an ammeter to see which switch operates which lamp.
It's all moot because any electrician installing 3 switches in the basement for three lights in the attic, would have installed 2, 4-ways in the attic stairwell just to jumble the order and keep things interesting. 2 of the basement switches would be able to be switched to control any of the three bulbs while the other could switch 1 of 2. It could be a great shell game for the kids. Joe