Translation:I worked for that company for three years.
No, "have worked" definitely implies that you're still working there (at least in this context).
Say you left Company A to go to Company B, and that was a year ago. You wouldn't say "I've worked at A for 3 years." It implies you're still there, which you're not. You could say "I've worked at A before" but I think the "before" explicitly forces it to the past. Maybe it's considered a different tense. Not sure.
Usually the -te form is used to convey the meaning of a resultant state. In the present tense "futtote iru/harete iru/kekkon shite iru" mean respectively "i am fat/it is sunny/i am married", and not in the sense of "i am being fat right now/i am getting married right now". Because it's the result (which is applicable to this moment) that matters.
So in the past tense, I'd argue that the past continuous isn't necessarily identical to -teimashita.