Are these correct sentences?

你好,
我不知道這些好不好。。

Where is the watermelon i bought?

我買的西瓜在哪裡?
我買的西瓜是在那裡的?
在哪兒是我買的西瓜?

謝謝你們。

你好:

Where is the watermelon i bought?

我在哪裡買的西瓜? ——这样说是对的。

不客气。

Hi tinkleburg,

I guess what you want to know is how to say when you realise you can’t find the watermelon you just bought, and you maybe asking your roommate. If my guessing is right, then you can say: “我買的西瓜在哪裡?”

example: " 老婆,我買的西瓜在哪裡?” 你有沒有看到? 奇怪了,我昨天才買的怎麼不見了。

"我在哪裡買的西瓜? " to me is more like "where did I buy the watermelon? (so it’s a bit strange to ask this question to yourself)

conversation example :
A: 你回來啦!手上拿什麼啊?
b:水果啊! 你猜猜看? 我在哪裡買的西瓜?
a:不是在每次去的超市嗎?
b:是在菜市場買的?

“在哪兒是我買的西瓜?” — not a correct sentence to me.

let me know if you need more explanation .

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Thank you very much, very clear!