There are a few ways to accomplish this. Here are two:
1. Have a "Today is date" trigger, set for the first of each month. Below that you could have a Wait set for Tuesday followed by a Wait for 1 day. Then, have a second Wait set for Tuesday, followed by a Wait for 1 day. Finally, have a third Wait set for Tuesday, and what you want to do on the third Tuesday of the month such as sending an email.
You can either add multiple Triggers across the top for each month, or simply change the date in the Trigger after the 1st of the month.
2. Create a Custom Date Field based on the date of the first email that needs to be sent. Then, integrate with PlusThis and use their Date Calculator feature. Then, simply input the calculated dates into your Campaign Wait Elements right above the send email elements.