Auto-Updating Option Chain Sheet

I have a Google Sheets workbook started and now want to automate three tasks inside it:

1. At exactly 15:00 hr every trading day the sheet must call the NSE option-chain endpoint (or another reliable public source) for both NIFTY and BANKNIFTY. From each chain please bring in these fields for every strike on both the CE and PE sides: Strike Price, Open Interest, Volume, Bid Price, Ask Price and Last Traded Price.

2. In the same trigger the script should pull the last-three-day Bhavcopy files and extract the CE ⁄ PE LTPs for the same strikes. Those values remain frozen as historical snapshots.

3. The sheet then calculates percentage performance for each strike over the last one, two and three sessions. No absolute figures are needed—just clean % moves, formatted to two decimals.

A simple Apps Script that runs on a time-based trigger is fine; if you prefer an external API connector that also works as long as it lands the data in Sheets automatically. Please avoid manual downloads or paid add-ons.

Deliverables
• Google Sheets file populated with the live and historical tabs
• Apps Script code fully commented and bound to the 15:00 hr trigger
• Formulas or script logic that output the three percentage columns per strike
• A short README inside the sheet explaining how to adjust the trigger or add symbols

I’ll consider the job complete once the sheet refreshes on schedule, all requested columns appear accurately, and the % calculations match NSE figures for a sample day.