While taxagent is virtually always right, and yes 2 could shift taxes to 3d who gets no benefit, I see additional points.
To be involved does't alway mean that one puts in similar amounts of physical work. Its quite possible that one co owner brought extra financing, credity worthyness, specific technical or marketing insight, name recognization or something else to the table.
Further , I do agree that depending on how things are managed , 2 owners could quite well do things so as get paid themselves and not pay the 3'd. And it might not be visible pay but "benefits" and reimbursed expenses and all that fuzzy stuff like cash.
You may want out because you see limited prospects---but that's a poor bargaining chip. If you posture it that you see a great future and you want to cash out to an ( in the wings) outside buyer who will take a much more aggressive role in firm --perhaps the two insides will take a special interest in not having a 3d hand in stew and cash you out on better terms? Its the value of an unacceoptable alternative that may count to them.
So in a sense its wise to stay current with how boooks are handled , keep copies, and especially how cash is handled..........