The basic rule fo changing 3rd tone if there are 2 or 3 consecutives 3rd tone is the following :
3 + 3 = 2 + 3
3 + 3 + 3 = 2 + 2 +3
If you have a sentence with only 3rd tones you first break the sentence into basic entities (noun, pronoun, verb or adjective+noun) and apply the basic rule to each entity.
For example :
我很想给你买几把好雨伞。
Wǒ hěn xiǎng gěi nǐ mǎi jǐ bǎ hǎo yǔsǎn.
我 / 很想 / 给你 / 买几把 / 好雨伞。
3 / 33 / 33 / 333 / 333
becomes
3 / 23 / 23 / 223 / 223
Wǒ / hén xiǎng /géi nǐ/ mái jí bǎ / háo yúsǎn.
