Neapolitan Bundt Cake is a fun Bundt, but now that cake mix sizes have been adjusted, the recipe might also need some adjusting. One thing that might work is buying two boxes of cake mix and weighing out just enough of the second box so that you have a total of 18.25 oz.
This recipe was developed back when cake mix came in 18.25 oz boxes. These days the standard size is between 15 and 16.25 oz, therefore the ratios are off.