As linguists, we aim to describe what we hear without stating what is good or bad or specifying how something "should" be pronounced.
The TH sound at the start of words such as three and thin is pronounced in different ways around the world. In most of the UK, USA, Canada, Australia and New Zealand, this sound is pronounced as [θ], but in London and Hong Kong it is often [f], in Ireland, New York, Singapore and Brunei it can be [t], and speakers from China and Germany may use [s]. Note that I have not suggested that any of these ways of pronouncing this TH sound is better than any of the others. They are just variant ways of realising the sound.
Of course, dictionaries will often list the "standard" pronunciation as [θ], which is a dental fricative (so the tongue is placed against or sometimes between the teeth).
However, an RBA pilot recently told me that the pronunciation of three and thousand stipulated in air-traffic communications begins with [t], and this is confirmed on Page 3, Chapter 2 of the CAP413 Radiotelephony Manual for pilots (see the image on the right; the full manual is available on-line here).
It is interesting that in this critically-important domain, the standard that is recommended for all pilots, even those from the UK and USA, is actually the pronunciation adopted by many Bruneians.
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