Is it permissible in Islamic law for a person to contribute their effort (including a monthly salary) to a commercial office, on the condition that they receive one-third of the profits but bear no share of the losses?

Combining a fixed salary with a percentage of the profit as wages in an ijara (leasing/hiring) contract makes the wage unknown, and thus involves gharar (excessive uncertainty). The Prophet, peace and blessings be upon him, forbade gharar and forbade hiring a worker until his wage is made clear to him. This is the preponderant opinion and the view of the majority of scholars. The correct approach is for you to be an employee in the establishment for a known, fixed salary.