Hopefully you discussed these options with the ROTC Battalion you will be in at USC before enlisting.
You will be working toward being a SMP cadet, that won't happen with you battalion until they offer you a contract, they have up until the start of your junior year to offer you a SMP contract. Since you are going to BCT and I assume AIT, you will be deployable until they offer you a contract, just make sure you are aware of that. The Guard has a scholarship they offer called a GRFD, it pays the same as the National scholarship but the biggest issue is that if you take a GRFD then you are locked into the Guard when you commission with no chance at Active Duty. Make sure as well that if you accept any tuition assistance from the Guard in your state that it will not tie you to the Guard when you commission.
As far as being offered a 3 year standard scholarship from your battalion, it has happened and if you are offered one you would need to get released from the Guard, not sure how easy that is since they will have paid for you to attend BCT and AIT. The other thing to remember is that scholarships are given by the battalion for both good performance and as an incentive to new cadets. You will already be committed to the Army through the Guard and they will be able to contract you as a SMP, there is not a big incentive on their part to offer you a traditional scholarship since you are already obligated. Even if you decide to leave ROTC you will still have your obligation to the Guard for 8 years as enlisted if you don't commission.
These are things you should have already discussed with the ROTC Battalion, if you haven't, you may want to give them a call before you go to MEPS or ship for BCT.