Hi Richard,
Well, there are some tone change rules of 3rd tone.
1) When a 3rd tone is followed by another 3rd tone, it changes to a 2nd tone,
E.g. 你好 ni3 hao3 - ni2 hao3 , 理想 li3 xiang3 ( ideal ) - li2 xiang3
2) When a 3rd tone is followed by a 1st, 2nd, 4th tone or a neutral tone, it's pronounced as a half 3rd tone - the first half ( falling ).
I want to mention a bit of neutral tone, there is something wrong with Belinda's comment.
There are certain syllables pronounced both weak and short, that's neutral tone without tone marks above them.
E.g. 妈妈 ma1 ma - mom, 爸爸 ba4 ba - dad, 姐姐 jie3 jie - older sister, 怎么 zen3 me -how
But,sometimes, neutral tone makes things different
Like 东西 dong1 xi1 - direction, dong1 xi - things/ stuff
地道 di4 dao4 - tunnel, di4 dao - proper
Keep up the good work!
Liz.