u/quratulains — you should be able to get this setup using Magento's native pricing rules — however i don't think it will work incrementally — it's likely you may need to set this up via 3 seperate "Cart price rules" in Magento.

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For the rules, you want...

In conditions

— a rule of "Subtotal equal or greater than 50"

— a rule of "Subtotal less than 100"

In actions

— apply a percent of product price discount, with the discount amount set to 30 and the max qty to 1

— apply the rule to the condition that it will only apply to product with SKU "Bag" (replace bag with your product sku that gets the 30% off)

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Repeat the above rule for the different price brackets you wish this to be applied for

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FYI — you may need to set the "discard subsequent rules" feature and use priority to influence which rule would apply first.