its a very vague question. any business has the potential to get you $3000 a month but it depends on many factors.
Investment - most businesses do really well with a sizeable investment upfront. if you have good investment you can also acquire a running business without having to go through a process. even a fast food outlet, a franchise of an existing company, a store, a fuel pump are some examples of businesses that people do regualrly to earn decent money.
Skills - do you have any specific skills that make you stand out of the crowd, such skills can help you earn decent money too, i have seen consultants of different types earning alot in service industry.
Time - every business needs time to get mature. its tough in the early days to get started and then keep it running. if you have enough time on hand you can pull it through.
Your location - every geogrpahic area has some opportunities that are specific to that area. in some countries tourism is extremely profitable while in some countries IT services are profitable and in some countries real estate business is on the rise.
it really depends on where you are located, what resources you have access to, what kind of man power you can get and what finances you have. all of these factor combine to answer the original question.
