Using Git bash for windows run the below commands to add kdiff3 as your git default merge tool, before running the below commands please make sure you have installed kdiff3 and the path is same like below, if different then mention that path and add in the global config.
git config –global –add merge.tool kdiff3
git config –global –add mergetool.kdiff3.path “C:/Program Files/KDiff3/kdiff3.exe”
git config –global –add mergetool.kdiff3.trustExitCode false
git config –global –add diff.guitool kdiff3
git config –global –add difftool.kdiff3.path “C:/Program Files/KDiff3/kdiff3.exe”
git config –global –add difftool.kdiff3.trustExitCode false