Population Estimation: 1000 Females, 25yrs-100yrs & 3 Kids/Female

[marian0567]

Base isolated population is 1000 females. Each is one is on average 25 and lives to be 100 years old and gives birth to three kids...sperm banks so there are no men in the first generation population.

2nd generation is 50%male and 50% female and each female grows up and has three kids. Everyone lives to 100 (for the sake of simplicity) this repeats for 1000 years so what is the population at the turn of each century assuming a new generation happens every 25 years?
This is not for school I made this question up for myself but I'm incapable of anything beyond basic arithmetic please help this has been nagging me for months and I can't get Excell to work.

 

[mfb]

For simplicity, assume every women gets 3 children exactly at age 25. And as 50% are male and 50% female (after the initial step), you can just assume everyone gets 1.5 children.
If you keep track of the number of newborns in every 25 year step, this problem should be easy to model in excel, but you can also find an explicit formula for the number of newborns in each step, and derive everything else based on that.