His book "J'Étais Tintin au cinéma" (French for "I was Tintin in the Movies") might give a clue as to the plot
Just came across this while doing a little site "house-cleaning", so thought I'd take a look at what M. Talbot did inded say about it in his autobiography...
...and the answer is: not much!
Not through reticence, he simply doesn't seem to have much information. He tells us that the possibilty of a third film was talked about for a long time, and that it was goint to "perhaps" be in India. He was given riding lessons while the talks went on, "just in case..."
However, as time passed, and he heard nothing, he wondered if another film was thought too expensive, or if the company simply wanted to replace him.
At the same time, his own life was moving on - he met his wife, and married her in 1964, and then in 1965 he went into the army (having already completed his National Service). So by 1967 he wouldn't really have been free for a film, even if he'd wanted to pursue an acting career, which he realized he didn't, and moved into teaching for the quality of life it gave him.
So, had the third film happened, whatever the plot, it is quite possible we'd have had a new live-action leading man - or, perhaps, we didn't get a third film because the company lost their leading man...?