1)Third gear is another way of saying mackin', or pimpin'. like- 3rd gear in a mac truck.
2) Third gear is that stage when a male and female are inbetween being just friends and "officially" dating
3) It may be used to describe overdone affection between a dating couple
4) It may be used as a synonym for girlfriend or boyfriend.
1)Hey, are you in third gear with Lindsey?
2) Greg, where is your third gear?
Wow! Matt and Jamie are really third gearing it.