You have already created the design for the 2nd order model and completed those experiments? If so, when you choose Augment design, specify your response(s) and your factors. Click the Augment button and JMP will give you a model window. Add the 3rd order term(s) that you want to the model. Note that when choosing the number of runs, it will include the already completed runs and always defaults to adding 8 more runs (or the minimum number to add for the number of terms added to the model -- for example, adding 12 runs because you requested 12 additional model terms). So you will want to put some thought into the appropriate number of runs to add to your existing design.
Keep in mind that the additional levels that are added are based on the levels already used for the 2nd order model. They would not necessarily be optimal for a 3rd order design that is built from scratch.
I hope this helps.
Dan Obermiller