You are probably looking at it correctly. Both places have their ticket prices structured so that the longer you stay the cheaper it is. You could upgrade those 3 day tickets to annual passes and stay an entire week at Universal with the only additional cost being the extra nights of the hotel room, if you moved down to a cheaper hotel you'd be likely be spending less than your Disney portion. Alternatively you could do only 3 days at Disney in a deluxe resort with 3 day tickets and probably be looking at spending much more than what you are spending on 3 days at Universal.
It is always cheaper to do one or the other, doing both breaks the bank!
My suggestion since you getting 3 day tickets is to look at getting them on
undercover tourist, they usually have a buy 2 days and get the third free going on. Then you can wait for annual passholder rates to come out on the hotels and try to get your room with that rate, you only need to upgrade one person to an annual pass to get the hotel rate though usually the seasonal pass is the same price as their 3 day tickets so you might as well upgrade everybody.