Unfortunately, if your father's stroke was 3 years ago, there's little chance of any major improvement in his symptoms. Therefore, you'll have to make the most of his current skill
level. In broad terms, it may be best to think of aphasia as encompassing two separate elements: the ability to understand, and the ability to speak or write. It sounds like your father is having more difficulty speaking and writing. In some ways, this is fortunate, because the inability to understand is more troublesome.
Given what you've said above, I think the next step would be to have him evaluated by a good speech therapist. A speech therapist specializing in stroke (available at many rehabilitation centers) should help tailor a communication method for your father that allows him to maximize his skills. This includes using tools such as a message board, which would allow him to communicate substantially more information than trying to write or type out his thoughts. For example, it's much easier to point to an icon that indicates, "I would like to go to the movies" than it is to type this information out. If you can't afford a therapist, you can create a board at home with pictures that he can point to, allowing him to convey many common phrases.