Hi there, I'm no basketball expert, so it may be a sports idiom like you guessed. As a native speaker however, if someone used this phrase to me I would assume the plan had three points to it. Either three stages or three different elements that work together.

1) I'd say "in" here: "Sometimes it’s difficult to make progress IN English." 2) At least for American English, you can say: "Sometimes, it’s difficult to progress WITH English". The difference here is that you are using "progress" as a verb and not as a noun (as in #1). It's pronounced differently. Here, you would pronounce it as: pro-GRESS. In #1/as a noun, it's pronounced PRO-gress. 3) In basketball, there is an outer "arc" where, if you shoot from there, you get three points instead of the usual two points. So a three-point strategy would involve making a shot from this area more often because you get an "extra point" for doing so each time. 4) "I need to clear a space on my desk for my new computer." - In this case, it can be both (with "a" or without). In this context, "clearing *a* space" specifies that *the* space you are clearning is being set aside especially for the computer. You're not just "making room" for a new computer (without "a").

I agree with Katherine. I'm an American and I love sports metaphors, but I would understand a "three-point plan" to mean a plan with three main components, not a basketball metaphor. If it were a great plan it would be a slam dunk!

Ah, I misread your question as asking what a three-point strategy was in basketball. Yes, a three-point strategy would involve three parts/strategies to execute the plan.