If you look at the tenses of each sentence, they actually all are stating different things and are not identical.

His job started a year ago.

In this sentence, it is stated that our undefined subject, “he”, started working at a particular factory one year ago. We only know that our undefined subject had worked at this particular factory for some time and that he decided to start this employment 1 year ago.

He has been working for this factory for a year now.

Our undefined subject has worked at this factory for at least one year and is also currently working at the factory. The “has been working” is a form of the present perfect continuous tense, which is used to denote an event that happened in the past and is still ongoing or is still applicable to the present.

He has worked for this factory for a year.

In this sentence is stating that our undefined subject worked at this particular factory for at least one year at some point in the past and that this information is probably relevant to the present. The difference in meaning between this sentence and the prior one is that “working” is the continuous tense form of the verb “to work”, while “worked” is the past tense form of the verb “to work”. We can assume that the third sentence is referring to a past event and not a continuous event because of the verb tense used, while it is still relevant to the present because of the present perfect tense of “has”.