What is meaning of -vvv option in cURL request

explanation : -v (--verbose flag) is useful for debugging and getting extra information about the response from server. Single v is just Enough.

From Curl documentation :

-v, --verbose

Makes curl verbose during the operation. Useful for debugging and seeing what's going on "under the hood". A line starting with '>' means "header data" sent by curl, '<' means "header data" received by curl that is hidden in normal cases, and a line starting with '*' means additional info provided by curl.

If you only want HTTP headers in the output, -i, --include might be the option you're looking for.

If you think this option still doesn't give you enough details, consider using --trace or --trace-ascii instead.

tl;dr: there is no difference between -v and -vvv.

Specifying -v multiple times usually means to increase verbosity accordingly.

This is true, e.g for a software like memcached:

-v            verbose (print errors/warnings while in event loop)
-vv           very verbose (also print client commands/reponses)
-vvv          extremely verbose (also print internal state transitions)

(behind the scenes the options parser accumulates the level of verbosity).

But with curl command-line tool this is not the case. As you can see from tool_getparam.c, passing -v simply toggles the so-called trace type to TRACE_PLAIN. Passing -vv or -vvv does the same.