Although it's certainly possible to get a lot out of these islands in three days, once you find the resort best suited to your needs, you won't want to lift a suitcase again for at least seven, and probably longer. Many resorts operate with seven-day minimum stays as the norm, and menus and activity schedules are created accordingly.
If You Have 3 Days
It's best to choose a resort you won't spend half a day getting to, so head to the Mamanucas. You can be checked-in and then check-out in a bathing suit by about noon, give or take a "welcome" drink. If you're interested in seeing more islands you'll find many of the resorts here offer island-hopping trips using their own small craft. Conversely, you can tackle these pristine beaches by day and retire to Denarau Island's international-standard comfort at night by taking a South Sea Cruises day trip that leaves daily from Viti Levu.
If you must visit the Yasawas for this short a time, a Captain Cook cruise departing from Denarau Marina is your best option.
If You Have 5 Days
Awesome Adventures has a five-day trip through the Yasawas, but it's still a better idea to unwind in one place. With this much time you can appreciate the versatility of a rotating menu and take a scuba diving course—the abundance of beginner dive sites and excellent visibility make these two groups fantastic places to learn. From the Mamanucas, consider a boat tour to Monuriki Island—the setting of Castaway—or a South Seas day cruise out to the Yasawas. From the Yasawas, organize a romantic day on a private beach complete with picnic lunch—perhaps to the Blue Lagoon, an adventurous swim in the ancient Sawa-I-Lau caves, or an invigorating hike through one of the island's lush, volcanic terrains. Whichever group you choose to reside in, there will be time for a village tour and perhaps to see a meke.
If You Have 7 Days
Seven days is the usual minimum amount of time guests spend in these parts. This gives you time to either fully unwind or take advantage of the many activities that a resort offers. Fidgety travelers might consider island-hopping with an Awesome Adventures "Bula Pass" and trying numerous budget or mid-range resorts.