I asked this question a while and it was explained to me in the following way:

When you have two third tones in a row 你好 you pronounce them ni2hao3.

This rule is always the case. Other exmaples are 我想 (I want), 我有 (I have), 你有没有 ni2you3mei2you3 (do you have) etc. You are best to learn the tones of these little groups rather than the tones of each word, because it's easier to say them aloud when spekaing when you don't have to think "oh, what's the tone of that again?". Obviously you want to know the tones, but you needn't pronounce them all really separately, it'll sound "disjointed" if you do that. 

When you have three third tones in a row, you get 2 2 3. Examples:

我很想去 - wo2 hen2 xiang3 qu4 - I really want to go 

我很喜欢看书 - wo2 hen2 xi3 huan1 kan4 shu1 - I really like reading books 

我有很多钱 - wo2 you2 hen3 duo1 qian2 - I have lot of money

You need to know how to "split sentences up" into little chuncks. That way, you know 1) what they mean and 2) how to pronounce them. Let me show you what I mean by first explaining this splitting up process. Here is a sentence from a book I'm reading at the moment. The slashes show you how you need to break the sentences up. I have tried to keep the chuncks short. You need to work on "seeing bigger chunks", it makes reading easier. 

当 / 玛丽 / 被 / 送到 / London / 庄园 / 跟 / 她 / 姑父 / 一起 / 生活 / 的时候，每 / 一个人 / 都 / 说 / 从来 / 没 / 见 / 过 / 长得 / 这么 / 不喜欢的 / 小孩。这 / 的确 / 是 / 事实。玛丽 / 有 / 一张 / 瘦削的 / 小脸，一脸 / 愁苦，一个 / 弱不禁风的 / 小 / 身板，一头 / 稀疏的 / 浅色 / 头发。

Literally translate this like this

At Mary bei4 was (this means by, but can also be used in grammar structures. 被verb means "have been" or "was". 我被坐牢 means "I have been/was imprisoned") sent to London manor with her uncle together live (the time of), every one measure person all said "never seen before grew this not charming small child". This indeed is true fact. Mary has one measure thin small face, one face anxious, one face extremely delicate small body, one head thin/sparse light colour hair"

Now, let's turn that into real English:

When Mary was sent to the London Manor to live with her Uncle, everybody said that they'd never seen such an unpleasent child. This, indeed, was the truth/ Mary had a thin face, a face that looked anxious, a small framed/delicately built body, a head of very thin, lightly coloured hair. 

This chunking is very important, because it should you how both how to read and understand things, and how to pronounce things. Now, back to third tones. You need to group them like I have done above, and the limit is 4 in a row

我也可以去 - wo2 ye2 ke2 yi3 qu4 - I can go too

我也想买东西 - wo2 ye2 xiang2 mai3 dong1 xi0 - I also want to buy stuff

Now, exampes of when you get a lot

你也想给我写信吗？- you need to break this up

你也想 / 给我 / 写信 / 吗 - ni2 ye2 xiang3 / gei2 wo3 / xie3 xin4 ma?

You can have two third tones next to each other, so long as it agress with the grouping. 

I hope this helps. 

Laurence