Use "since" in its meaning of "starting in the past" with the present perfect form of the verb (have + past participle) or the present perfect continuous (have / has been + "ing" form of the verb). Your example would use the present perfect tense.
Of the three examples you give, #3 is correct for its use of "since". Your involvement began in 2015, and is ongoing.
(Incidentally, we would say "the telecom industry..." and we would not capitalize "Telecom Industry" as it is a generic term. We would only capitalize proper names, i.e. if that were the actual name of a specific company.
So correct would be: We have been involved in the telecom industry since 2015.)
Your other examples:
The first example is not idiomatic, although the meaning would be understood. "Since" does not correctly go with the simple present tense, for the reason given above. To rework this sentence, you could say, "We are involved in the telecom industry, and have been since 2015." It's a bit redundant, but it works.
The second example is not grammatically correct at all - If I were editing this, I would simply understand that the writer had omitted "been" by mistake.
Hope this helps.