Dependent Care Credit only $600 applied by TurboTax, actual expense is $8k, so shouldn’t $3k applied instead?married filing joint with 5 yrs old child as dependent

There is a phase-out range for the Child and Dependent Care Credit.  If you did not get the full credit amount, it may have been reduced by the phase-out.

To see the phase-out limits, take a look at the following link:

IRS Publication 503 Child and Dependent Care Expenses