Most teachers will suggest you practice and it will come, while not being too worried in ordinary conversation. "Dat ting" (for "that thing") is perfectly well understood in general, and in particular, if that's the "common mispronunciation" of speakers where you come from, native speakers will be quite used to it and usually understand very well.
My suggestion for practice would be
- Study a video and tongue position guide to learn how to pronunciation of /θ/ and /ð/
- Check with a native speaker that you can say it perfectly when you do it very slowly
- Practice very slowly: speed will come later
It's very important you do it slowly and correctly. The usual problem is that learners try to do it too quickly without ever being sure they're doing it correctly. Correctness comes from learning slowly, speed comes with practice.
Remember when listening that many native speakers will not pronounce some of your examples correctly, especially when speaking quickly. In particular, many will drop the ending of "sixths" and "three-sixths" is barely distinguishable from "three-six". If maths is your subject, however, it's probably best if you can master them.
How to get by
If you can't say "maths" and be understood because they think you're saying "mass", "mats" or whatever, say "mathematics" which will be understood whatever sound you make for "th".
If you can't make "three-sixths" sound right, add an "e": "three-sixthes", even "three-sixethes", "hundredthes" etc. It's not elegant, but it will work! (It's a very common "mistake" to insert extra vowels, especially for native Italian and Japanese speakers.) Or explain with "three over six" which every numerate person will immediately understand.
As well as the other excellent answer about Estuary English ...
Many speakers don't make /θ/ and /ð/ and are perfectly well understood.
- Many French speakers of English use S and Z sounds ("Za sing")
- Many West Indian native speakers use T and D sounds ("Dat ting")
- Many Estuary native speakers use F and V sounds ("va fing")
- Sometimes the sounds are dropped or replaced by glottal stops (even harder for non-natives to say)