Thetatismus is apparently a term used in speech therapy to describe the inability to pronounce one or both of the dental fricatives: /θ/ as in three, and /ð/, as in these.

I came across it in Anthony Burgess’s book A Mouthful of Air – Language, Languages … Especially English.

So now I know what I ’suffered’ from when I was a lad – I couldn’t pronounce /θ/ and didn’t know there was a difference between /θ/ and /f/, so didn’t distinguish between three and free. I also substituted /v/ for /ð/ when it appeared in the middle of words. Even now pronouncing /θ/ is not entirely natural for me and I tend to say /f/ instead when I’m not concentrating.