ということは・つまり・というのは: the difference
ということはN2
Meaning
That means, That is to say
Past papers
Example
末っ子ということは、本当は甘えん坊なんじゃない？
You're the youngest, so doesn't that mean you're actually a bit spoiled?
つまりN3
Meaning
In short, To sum up, In other words
Past papers
Tested once in past papers
Example
明日何時でも良いんだって。つまり、夜遅くなっても良いらしい。
They said anytime tomorrow is good. To sum up, it seems that even late at night is ok.
というのはN3
Meaning
~ Known as ~ is, ~ Called ~ is, ~ Means ~, Because
Past papers
Example
雪というのは、自然現象です。
Snow is a natural phenomenon.
How to tell them apart
「ということは」: draws a new conclusion from the information just given. 「店が閉まっている。ということは、今日は定休日か」 (The shop is closed. So today must be its regular day off).
「つまり」: restates in other words, 'in short'.
「というのは」: gives the reason, 'the reason is ...', often ending in 「～からだ」.
These three are easy to mix up in fill-in-the-blank questions.