1. 재밌잖아.

It's fun.  ----------->  Just because it was funny.

=> Another way: Your sentence is okay and this is another way of saying it.

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2. 안녕, 주말 잘 보냈니?

Hi, did you have a good weekend?  ---------- >  Hi, did you have a nice weekend?

=> Another way: Your sentence is okay and this is another way of saying it.

3. 물론 잘 보냈지.

Of course, I did. ✔​ ​​​​​​​​​​​​​​​​​​​​​​​​​​​​​