This help page explains our Discount settings:
I would suggest setting the discount for All Cart Items, taking a $30.00 Flat amount off the Item Total, with Min. Item Quantity = 3. Possibly also set Min. Item Total = 30.00, if you don't want item totals under 30.00 to be discountable to "free" plus Shipping. You can also enter an Expiration Date if you wish.
If you will be issuing each Groupon buyer a unique code, you can paste in a list of Discount Codes (up to 10,000 codes if you don't have any other discounts), and set Max. Use Count = 1 (so each code can only be used once). Submitting these settings will create a series of identical discounts, one for each code you pasted into the Discount Codes list.
This help page explains our Shipping calculation settings:
We can provide more specific instructions if you clarify whether you just want to charge the same, flat shipping rate per item for all products, or a per-unit rate that differs among products, and whether you will need differing domestic vs. international rates.