Working Days. help Required please

Hi Forum buddies.

I am creating a manufacturing leadtime sheet to indicate Ex work dates of products from my company along with delivery dates.
How do I get the date formula to only calculate working days ?

I.e

Ex works date 13/12/2018
Transit Time 3 days giving s theoretical delivery date of 16/12/2018, but in this case the 1st available working day is 17/12/2018

Ive tried various formulas, WORKDAY etc but cant seem to get it right.

Any assistance would be greatly appreciated.

Regards

140A