If you ask Arabs they pronounce each Of these letters different ( u may even find many Persians pronouncing them different )
there are some facts that may help you :D
1.even as a native Persian speaker I don't notice these differences easily when Arabs pronounce these different letters , you may notice ث is "th" and س is "s" , but we pronounce all of them like "S" .
2. It is quite important in Arabic to pronounce them correct . because these little differences can change the meaning of sentence or words easily ... and when it comes to some holy religious texts and etc you should take care how you pronounce things in Arabic :D
3. knowing Arabic besides Persian may help you with the dictation . for instance Arabs have some roots for their words which came in 3 letters like : ظلم ,فعل, ظهر then they turn these roots into words wich based on their specific structure can accept different grammatical roles in sentences .
for instance ظالم فاعل ظاهر as subject, or مظلوم مفعول مظهور as objects ...
so if you know some of these roots (for instance lets say 50 of them ) and some of these word structures (like 5 or 6 ) then you can find out if a word is originally Arabic or Persian and this can help u have less problems with dictation .
for instance I face a new word like : مسدود /مردود/محدود/مسبوط/مسکوت as you see all of these words share the same structure and you may find their roots easily . in the case of مسبوط you can conclude that this word is based on مفعول weight (وزن as they say to these structures in Arabic ) so the root letters of ف ع ل in مفعول can match the س ب ط in this case and the root for this word is سبط so you won't have any problem knowing the correct dictation anymore .
4. you face a new word like :راز which means secret . from your Arabic background you know this word can not be among Arabic words and It is persian ... so it is very less likely for it to be written like : راض or راظ or راذ
because original persian words usually use :ت ,ز,س