Each third must not infringe on the others to keep you healthy. Work must not take over and instead provide the means to make the most out of your personal time. Neither work nor personal time should threaten the time you spend taking care of yourself. Moreover, work is necessary to sustain your life and activities. Keep the three in equilibrium and life will be good.
No, you do not have to spend an equal amount of time on each to keep the three in balance. If anything, you should maximize personal, soul-enriching time as much as possible (it often takes a lot more than you realize to truly balance out your work). But you can definitely add up the hours in a week to determine whether your work/life balance is out of whack or not.
Keep your time in check. Keep your life in balance. Juggle the three balls well and you may yet find happiness.