How do I automatically subtract 50 rupees on every third buy of same product by the same customer in ASP.NET C#

We can't answer that: it's just too vague and unspecific.
The problem is that you need to think of this as a whole system: you need a way to track - and identify - customers so that you can store their "hair cut history". Until you have that sorted, you don't have a way to do this in an electronic form.

So start by thinking how you would handle it manually: You'd issue each customer with a "loyalty card" which had spaces for a stamp. Each time the customer came to the shop, they present the card, you cut the hair, stamp the card, and if the number of stamps is a multiple of three, you give them a discount.

That's not a bad model for a computerised system, but you need to start with "identify your customer" - so they need to log in to a system. Which means you have to store customer details - username, real name, password, email, etc. and write a login system to match them us and prevent fraud (you don;t want to hand out discounts they haven't earned, do you?). And that means storing it all somewhere - a text file, a database: I have no idea what you are familiar with so I can't make suggestions.
Then you need to expand that to record hair cut details - a note of when they had a hair cut so your can check "how many stamps" they have. Again, your current skill set will dictate how you store this.

So think about how you are going to store the data, and work back from there.