Typically, if your speaker has only one input, and is tri-amped, the the crossover is in the cabinet. (Or 2 crossovers in your case) So, for instance, you would use the external crossover to split the sub freq's from the mid/high, send the subs to dedicated subwoofer and the mid/high to the speaker cab via your speakon connection. The mid/highs would then be broken down in the cabinet to their respective speaker.
What speakers are you using? That would give us a better idea of what you have.