If I completed my Bachelor's degree in three years, have I "earned four years of college credit" or not? I just completed a second degree and would like to use the AOTC.

Yes, you should be able to claim the American Opportunity Credit, as long as you:

• Are pursuing a degree or other recognized education credential
• Are enrolled at least half time for at least one academic period* beginning in the tax year
• Not have finished the first four years of higher education at the beginning of the tax year
• Not have claimed the AOTC or the former Hope credit for more than four tax years

From: https://www.irs.gov/credits-deductions/individuals/aotc