how to exactily repeat the n matched pattern in result string

Try this:

:%s/^+*/\=repeat('   ',strlen(submatch(0)))/

submatch(0) contains all the matched + at the start of the line, strlen counts them. So for every plus sign at the start of the line three spaces are inserted using repeat.

For more information:

:help sub-replace-expression
:help repeat()
:help submatch()
:help strlen()

An elegant substitution command for this case is the following:

:%s/\%(^+*\)\@<=+/   /g