Hello, I wanted to ask if I should just buy BTC/LTC/ETH right away when I get the money, or should I wait a couple of weeks to see if their prices go down?
Consider this. There are two events happening that may earn you an equal amount of bitcoin in a different version cryptocurrency. The events are called hard forks. Hard forks result in airdrops of a new currency. They have to do this if they want the new (and "improved" so they intend) version of Bitcoin to exist and thrive.
If you buy more Bitcoin now you will receive more airdropped coins of the two fork currencies coming up on Oct. 25th and mid-November.
Just something to consider.