Mobile Companies introducing new models for every 3 months and forget their s/w upgrade on old products. Are they cheating us or we getting cheated?

Basically, it is part of the marketing process of a company. Since competitors are always updated, the company uses update and introduce new products for them to hold their users.

NO ONE IS CHEATING -  TECHNOLOGY IS GETTING UPDATED - OLD VERSION MOBILE WOULD NOT BE COMPATIBLE WITH NEW -  IF IT IS COMPATIBLE MEANS THEY WOULD GIVE OPTION TO UPDATE

They are marketing. Simple as that. Here and there little changes, one has to be smart enough to invest money especially in smartphones.