Magento2 how to offer buy x @99 ?

Please help me for this:
As i have one category in which products are of different prices, i want to give combo offer for that category like buy 3@99.
If customer add products either 1 or 3 max then subtotal of the cart will be 99 only.
Please guide me how to achive this in magento2.2.5
I need this urgently for marketplace website,