Am I able to renew a 3-year AppleCare+ plan?

The original paid upfront plan has always been a 3 year plan; if you renew it, the plan then becomes an annual plan. See the information in Continue your AppleCare+ coverage - Apple Support including the following:

• If you paid upfront for an AppleCare+ plan for your Mac, you might be able to purchase new coverage that renews annually.

Note carefully in the article that @FoxFifth pointed out you can only renew within 30 days after the original AppleCare+ has expired. Beyond that 30 days it is no longer possible.