I have a T5008 that has all info to determine a Cap Gain/loss on a disposition. Does T-Tax do that automatically or do i have to enter that separately?

If all the information is shown on your T5008 slip, you do not need to enter that information again in the Capital Gains and Capital Gains Deduction profile.

When the T5008 slip is entered to report a security transaction there is no need to report the transaction in Schedule 3, as this would be reporting the transaction twice.