Getting to third base is the most ambiguous stage of high-school heterosexual interaction. The definition really depends on how far the boy goes: is it an intermediate stage followed by going all the way, or is it the final stage of making out? (everything that follows is from the boy's perspective) When I was a kid (mid-70s) it meant groping skin-to-skin instead of through the clothes. Today it's gone farther: "getting to third" implies that you eventually squirted; "I got as far as third" means that you didn't do it inside her (hand job, blow job, dry-hump, just lose it while making out, etc.). If you go all the way, it doesn't really matter when you got to third.
"How far did you go?"
"Third base, dude! She wouldn't drop her panties, but we kept at it until I spooged my boxers."