2000blazer brake lights stopped working changed bulbs but only 3rd brake light is working whats wrong?

Something no one has bothered to mention yet is your actual brake light switch. The third brake light has it's own set of contacts with-in the switch - seperate from the main brake lights which also are used for turn signals. Obviously, you should first check ALL your fuses, not just the ones marked as brake lights. On some vehicles (Dodge Neon for example) the third brake light runs off of the A/C fuse. Weird, but true. However, it all the fuses check out ok, I'd try a new brake light switch,,,, they're inexpensive, easy to change, and prone to failure. (One of the most common causes of failure I've found). Hope this helps to get you on the right track.
2000blazer brake lights stopped working changed bulbs but only 3rd brake light is working whats wrong?
you need to check the ground wire on the back of the harness for this one,the middle one will still work because its on a separate ground wire,but when they loose the main ground in the rear they wont work at all,once you locate the harness follow the wires you,ll see the ground wire,its real thin on these,and breaks off real easy,good luck hope this helps.
2000blazer brake lights stopped working changed bulbs but only 3rd brake light is working whats wrong?
It could be a blown fuse or a wire short of some kind, to test this get a test light from a neighbor or auto zone (like 6 bucks), and take out the light bulb and in the socket put the test light and if it lights up your having problems with bulbs blowing out, but if it doesnt light up it could be a bad plug or wiring. Best thing if you dont want to mess with it take it O reilly's or Auto zone and i think they can check it for free. Good Luck
Sounds like a fuse-

the third brake light is on a different fuse line than the others.

Check your fuse box and replace the dead one.