It is quite an achievement!
I have outlined my understanding of the connotations of following words with similar meanings, but I don’t know if they’re correct.
成功: success; emphasizes the result, the fact that the effort succeeded, rather than the nature of the achievement.
功績: achievement; emphasizes the nature of the deed, rather than the result (the fact that it was a success). thus, it also might refer to a contribution (to society, a bigger cause), not limited to oneself.
成績: score; grade; brings to mind a number / rating / measurement of achievement.
Your definition of 成功 and 成績 are correct, but 功績 is incorrect.
Both 成功 and 功績 emphasize the “result”. 功績 is closer to “contribution to the society/organization” or “legacy” (that will continuously bring benefits to the society/organization for a longer term).
By the way, the original Japanese sentence is unnatural. It even sounds like a sarcasm: “Hahaha! What you did is just trivial. Don’t trumpet it as your personal achievement”. たいした shouldn’t be used in this English context.
I would translate the English sentence as:
(very casual) すごいじゃん！
(very casual & masculine) スゲーな！