grammar - The differences between ～がたい、〜にくい、〜づらい

I was wondering what the differences are between these three: ～がたい、〜にくい、〜づらい

They all seem to be some sort of variant of "Hard to do ~". But it seems they are used with different verbs and/or imply slightly different things. However, I'm not sure on what these are.

Questions:

1) When should you use one of these over the others. For example, when should you use ~がたい over ～にくい and ～づらい

2) What is the difference (and are all 3 acceptable?) between:

1. わかりがたい


2. わかりにくい


3. わかりづらい