How on earth did you decide on that being your main system? Those are mismatched main sizes (asking for problems) and a VERY small system to be considering a Digidesign Venue for. In fact, that setup speaker wise looks an awful lot like a drum monitor or side fills to me.
$3000 is not a lot of budget to get a PA on, so there will definately be compromises that have to be made. The first thing I would do is prioritize. Make a list of all of the things that your system HAS to do. How many channels? Do you HAVE to have monitors for that budget? How much output power do you NEED, how much coverage (primarily width) etc... This will help you to decide what components you need to buy. Now you can decide how much money to spend on each. Someone above mentioned sticking to JBL and EV. This is not a bad idea, but for budget stuff, I would definately look at the Turbosound TXD line. In my opinion it holds its own against the JBL and EV products that cost double. You may also want to look used. As for speaker sizes, there really is no formula. Every different driveer has different power and frequency specs regardless of the driver size. To say that you need 18" subs seems pretty silly. Why on Earth would you need 18" subs? whats wrong with 15" subs? or 12" subs? It all depends on the driver, the box, the amp, the crossover, the design etc... as to how one sounds compared to another. I will take my RCF dual 15's any day over some yorkville dual 18". The RCF's will be clearer, seem louder, be more defined, and hit harder. EAW also makes an awesome 2x12" sub, as does Nexo, Meyer etc... There is no reason that your low mid driver can not be bigger than your sub driver. As long as each is tuned and powered properly, there will be no problems.
