With itemized deduction, which includes mortgage interest and some amount for property tax, car registration tax. Why turbo tax calculates it as $10000

Starting in 2018, the SALT deduction (which is either state/local income tax plus property tax OR sales tax plus property tax) is capped at $5,000 for married couples filing separately and $10,000 for all other filers.   Mortgage interest is not part of the SALT deduction.   This TurboTax FAQ explains the limits on mortgage interest, if applicable.  

View your Schedule A to see the details of your itemized deductions.    More information on SALT and other itemized deductions.

How to Maximize your Itemized Deductions

Since SALT deductions are capping property tax and other taxes to $10,000 for married filing and if I added $3000 only for property tax, why I am getting full $10,000 credit here as part of itemized deduction.

Income tax is included in SALT as well.  This would include tax reported on you and your spouse's W-2s in boxes 17 and 19.