It is grammatically correct either to use only a single article before the first noun or to use an article before every noun. I would recommend using the article before every noun if you would like to emphasize that you were employed in several roles (whether or not these were at the same time) and to use the article only before the first noun if you would like to emphasize that you were employed in one multifaceted role.
I would also not recommend using a colon, as this is a rather simple list. Instead, you can simply write "I was employed as a declarant..."
I am not quite sure what you mean, but I do not believe "declarant" is used in this way in English. (Instead it has a specific legal meaning.)