Need help - Bash script for file manipulation

[Relar]

I'm kinda new at scripting, and I have the basics down, but here's a question I couldnt find an answer to:

I have a long text file full of numbers (around 400). I have a script to sed the file so each line is a unique number. Now I want to add every 3 numbers.
So that if I have, lets say, 99 numbers in the file, my output will be 33 numbers (sum of num1+num2+num3, sum of num4+num5+num6, etc..).

Do I need to map the file into an array or is there some simple while-do loop for this?

Thank you!!

[ghostdog74]

awk '{ s+=$0}NR%3==0{print s ;s=0}' file

s+=$0 means addition of each record BUT when NR%3==0 (means every third line. ) is hit, print out the sum and set back to 0

[Relar]

Thank you!!

Would you have time to explain what you did there?
If not, thank you again, much obliged

[nopycckn]

awk is very useful.