What is the difference between 钱 and 金钱 (and 款)?
In terms of usage here are the common use for those words.

金钱 = Usually represent wealth in writing. 

時間就是金錢 (time is money)
我的金錢 (my wealth)


钱 = Money
Most common form of money

多少钱 (how much)


款 usually means fund (in terms of money)

for example: 
(款)
信用卡还款 -- credit card transferring (funds)
退款- refund


钱 is more informal than the other two. (In general if there is a single character form and a two character form, the two character form is more formal.)

款 is also used in 贷款, which means loan. Generally 款 does not imply money that you have so much as simply money that you are using. 款 can in some situations have a negative connotation while 钱 is neutral.