Hello and Thank you for your answer!

It really works, but not for 100%

After making changes in carousel.less i can see the result on my page only if i refresh bootstrap.less (save without changing)
Tried to find answer in google, and it seems like it happens because of cashing (all files linked with @import in bootstrap.less) but there is no way to solve it for joomla 3

I am looking forward for your answer

Hi Proton,

It is yes, that you should not change the file in that folder,
You can copy that file to your template less folder, then change bootstrap.less in less folder to included your file.

For example:
//@import "../../../plugins/system/t3/base-bs3/bootstrap/less/carousel.less";
@import "carousel.less"


And we also recommend you to compile the LESS to CSS one you are finish making changes.

For T3, when you change something, you should turn on the Development Mode.
And Compile LESS to CSS once when you turn off Development Mode.

Regards