“Ben oraya üç sefer/kere/kez/defa gittim.” (Meaning: I have been there three times). But sefer/kez/defa also have the meaning “this time” “Bu sefer/kez/defa aynı hatayı yapmayacağım” - Meaning “I won’t make the same mistake this time.” In this sentence, you cannot use “kere”. It only has the meaning “times” (e.g. three times) “Kere” is also used for multiplication. “3 times 5″ means “3 kere 5” in Turkish. While I am positive that you cannot use “sefer” instead in this context, despite the fact that a native speaker would understand if you used “kez” or “defa”, I have never heard their usage in this context either.