I’m doing this project for a company I’m working on / at / for.
Which one should I use?
I’m doing this project for a company I’m working on / at / for.
Which one should I use?
A native speaker would use for.
The difference, I would suggest, is as follows.
I'm working for XYZ.
Here, company XYZ is spoken of as a group of businesspeople for which one may work. You could also be working for an individual.
I'm working at XYZ.
Here, company XYZ is spoken of as a location at which one may be and work. You cannot work at an individual in this sense of the word at.
I'm working on XYZ.
Grammatically, this is perfectly fine. However, one normally works on a project, a set of tasks, a plan, or some kind of issue. If you are an entrepreneur and play an active role in building a business or managing the core aspects of its development, then technically you could refer to it as your project and you would indeed be working on company XYZ. I do not believe this to be the case from the limited context provided in your post.
As an employee, you work for a company, you work at a particular location and you work on whatever tasks are assigned to you by your superiors.
All three are possible, but they all mean different things. Since you’ve not told us what meaning you’re after, it’s not possible to say which you should use.
You are working for a company if it is your employer. “I work for Bloggs plc”.
You are working at a company if it’s not your employer but you are physically working at its premises rather than your main place of work, probably because your actual employer is providing professional services of some kind to it. “I can’t come to the meeting tomorrow, I’m working at Bloggs plc”.
You are working on a company if you have a different employer but you are engaged in work related to the company, for example because you are an accountant working on an audit. “I’m working on Bloggs plc this week.”