The strict answer to your question is that 3 courses (at 4 Willamette credits each) are considered a full-time load. But it is not quite that simple — to make normal progress toward graduation a student needs 28 credits after the first year, 60 after the second, 72 after the third year and 124 to graduate.
There are two options to this question:
Take the normal four-course load for two reasons:
You are obviously very capable and should have no difficulty taking four courses.
Having a few extra credits when you start gives you the freedom over your Willamette career to take courses you would not otherwise be able to take if you had to take those core classes at Willamette.
Enroll in only three courses if you wish, given the transfer and testing credit you’ll bring with you. Apart from fulfilling various specific requirements related to general education and your major, you'll need to complete 124 credits to graduate. Divided among 8 semesters, that yields 16 credits per term, with one term at 12 credits. Thus, even if you weren't bringing in credit at entry, you would have one semester in which to take only 12 credits.