First we need to clarify, Your formulas are .45%, .40% and .35% not 45%, 40% and 35% Then you are dividing them all by 12.
In Addition
A1= 46,000,000 = (A1*.0045/12) <---This should work properly
A2= 146,000,000 = (A2-100000000)*.0040/12 <---This is only going to give you .4% of 46000000/12
A3=246,000,000 = (A3-200000000)*.0035/12 <---This is only going to give you .35% of 46000000/12
The below updated formula takes a number and breaks it up, you get .45% of the first 100Mil .4% of the next 100Mil and .35% of the remainder. Then I've divided by 12 as it appears you are looking to do.
=(0.0045*(IF(A1>100000000,A1-(A1-100000000),A1)))+(0.004*IF(A1>200000000,100000000,IF(A1>100000000,A1-100000000,0)))+(0.0035*IF(A1>200000000,A1-200000000,0))/12