I've been been working for "X" FOR three years. Don't use the present continuous.

“For [a period of time]”. “Since [a point in time]”. Then, yes, use the present perfect continuous if you are talking about the period of time. “I have been working at Company X for three years.” If you use “... since 2016”, “I have worked at Company X since 2016” or “I have been working at Company X since 2016”.

Hi Kia. As far as I understand this topiс, if you want to make an emphasis on your work experience you have to say, "I've been working for 3 years." or "I've been working since last summer." Moreover, this sentence means that you are still working at the company.

'I've been working with "X" for 3 years', if "X" is a colleague. 'I've been working for "X" for 3 years', if "X" is an employer or boss.