I don’t understand what part III ON FORM 8453-OL

Yes, part III of Form 8453-OL is used to make California Estimated payments for the current tax year (2024). 

Generally, you must make estimated tax payments if you expect to owe at least $500 ($250 if married/RDP filing separately) in tax for 2024 (after subtracting withholding and credits) and you expect your withholding and credits to be less than the smaller of:

• 90% of the tax shown on your 2024 tax return; or
• 100% of the tax shown on your 2023 tax return including Alternative Minimum Tax (AMT).

Taxpayers with 2023 California adjusted gross income equal to or greater than $1,000,000 (or $500,000 if married/RDP filing separately), must figure estimated tax based on their tax for 2023.