Can I place trailing Stop Loss on My Holding in Zerodha?

This is not possible as it comes under algo. You can place Stoploss using GTT order and manually trail it.

Once i place GTT order can I cancel it?

Yes, you can cancel GTT anytime you wish to.

What are the steps to cancel GTT?

Is it possible to TRAIL STOP LOSS in Options Trading?

But can this feature (trailing stop loss) be added as part of alerts then?, IMO placing automatic orders based on the trailing SL can be considered algo, but will it still be considered as an algo for sending alerts too?

I mean, let’s say i want to be notified when a stock is trading 3 % below it’s previous close price, and i want this previous close price to be trailed until the condition is met. Is it possible to have such a feature as part of the alerts ? Will this be considered algo too?

Because it is just an alert, based on the alert i am going to be the one to manually place the sell or buy order.