I have 3 operation systems in my computer, how can I delete 2 of them?

windows key+R

diskmgmt.msc

right click on the partition you want to delete, select delete

You might have to click on the partition that's left to increase the volume left from the two deletions

I don't know cause I never Installed 3 sets of windows on my computer.

Disclaimer: I am not a computer technician, I play one on the Interwebs.

If you screw it up..... I'm not responsible

Which three versions of Microsoft Windows have you installed on your computer? Add this information to your question.

Which version of Microsoft Windows do you want to keep? Add this information to your question.

You can delete your oprating system through the formatting method.

First of all, set your boot system, Insert ur boot cd in disk drive, Now format it easily.

I don't think you understand what you did and we need more to go off of then that. Take a picture or something.