I learned at school that "police" is a plural noun. But there are many different police. When we talk about secret police, military police, and riot police, can we say 3 polices?
You are correct that the noun "police" is treated as a plural.
It isn't an uncountable or mass noun. If it were, we could say things like "not much police" and "only a little police", like we can with "salt", "mud", "rice", "rain", etc. Instead, we say things like "many police" and "few police", like with any other plural noun.
Longman's A Comprehensive Grammar of the English Language (Quirk et al., 1985) refers to "police" and "people" as "unmarked plural nouns".
The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002) refers to "police" as an "uninflected plural-only noun". Such words are said to be "quasi-count nouns" because they cannot be used with low-number numerals. Hence, we can't say "four police", whereas we can say "two hundred police".
You can't say "polices". You could say "types of police", "kinds of police", or "police forces" (but the regular or civilian police may itself consist of multiple forces, e.g. different forces for different localities, and so "types of police" may be clearer).