One can express your sed in less Leaning Toothpick Syndrome[1] via:
find . -type f -name "*.html" -exec sed -i '' -e 's|\.\./\.\./\.\./|../../../source/|g' {} +
Using "/" as the delineation character for "s" patterns that include "/" drives me batshit - almost as much as scripts that use the doublequote for strings that contain no variables but also contain doublequotes (looking at you, json literals in awscli examples)
If your sed is GNU, or otherwise sane, one can also `sed -Ee` and then use `s|\Q../../../|` getting rid of almost every escape character. I got you half way there because one need not escape the "." in the replacement pattern because "." isn't a meta character in the replacement space - what would that even mean?
find [...] - exec [...] {} +
as opposed to
find [...] - exec [...] {} \;
worked fine and was performant enough for my use-case. An example command was
find . -type f -name "*.html" -exec sed -i '' -e 's/\.\.\/\.\.\/\.\.\//\.\.\/\.\.\/\.\.\/source\//g' {} +
which took about 20s to run