a crossover filters out specific frequency ranges to the right speakers (woofer, midrange, and tweeter).
example in this picture you have the midrange, the 1" tweeter, and the square thing is the crossover.
having a crossover will send your highs to the tweeters, you mids to your mids, and lows to you subs which will eventually help the sound of your bass. seeing that you said you are not getting the bass from your 3 subs.