If there are two third tones in a row, e.g. ni3hao3, the first one is pronounced as a second tone, i.e. ni3hao3=ni2hao3. At least, that's what I was told in a course. What happens with more consecutive third tones, e.g. with wo3 hen3 hao3?
The classic sentence for multiple tone 3s is “Old Li buys good wine.”
老 李 買 好 酒
In citation form they are all tone 3s, of course:
lao3 Li3 mai3 hao3 jiu3
Which ones change to tone 2? It basically depends on how fast you are speaking. If you are speaking slowly and carefully, only the tones that are within a phrase will assimilate. Here these are “lao Li” and “hao jiu”. So in careful speech it would be:
lao2 Li3 mai3 hao2 jiu3
At a faster speed, you will have two tone 3s in a row assimilate, resulting in something like 2-2-3-2-3 or 2-3-2-2-3. And at a really fast speed, all of them except the last will be affected:
lao2 Li2 mai2 hao2 jiu3
For more information see the linguistics thesis in the link below, starting at the bottom of page 6.