Would have been better as a TV series.
This film would have worked better as a TV mini-series. Nine episodes of one hour each would have covered much more of the material than the three films did.
A TV series based upon the younger of the Song girls is coming soon; XO Kitty. It will star the actress who portrayed Katherine (Kitty) Song Covey.
My comparison of the film to the book is here;
http://thatwasnotinthebook.com/diff/to-all-the-boys-i've-loved-before