Cracking little film.
'The Three Lives of Thomasina' is very cute and surprisingly existential. It's well shot and paced, with a terrific adult cast - the child members do cheapen things a little, but not to point where it affects enjoyment.
Patrick McGoohan (Dr. MacDhui) and Susan Hampshire (Lori) are both excellent, they suit their respective roles perfectly. Laurence Naismith, who I enjoyed in 1961's 'Greyfriars Bobby: The True Story of a Dog', appears in a somewhat minor role but still makes it memorable. Elspeth March does a fine job voicing Thomasina, too.
It sends a positive message and actually delves relatively deep into the human/animal side. Not saying it's a masterpiece in that regard or anything, I just didn't expect it would have that sorta spirit.
This was technically released in 1963 ('properly' in 1964), a year that I didn't enjoy in terms of Disney live-action films so this is easily the best in relative to that. If you love cats you'll enjoy this, you should give it a watch regardless.