Dear N:

You wrote:

Do these mean:  

a-They had robbed the bank and they knew about it.

b-Someone had robbed the bank and they knew about it.

c-They knew how the bank could be robbed.

d-They knew how they could rob the bank. 

1-They knew about robbing the bank.

 b is most likely, though somewhat ambiguous. Gerunds are not usually used this way. Normally one would use a clause to make it clearer.

2-They had information about robbing the bank.

 c, but again, a clause would normally be used.

3-They had insight into robbing the bank.

 d, possibly c (insight suggests something more personal).

All of these could vary slightly depending on context.