In this paper the author present applications of a trilingual corpus in language research. Comparative and contrastive studies of Polish and Bulgarian as well as Polish and Lithuanian have been already conducted, but up to the best of authors' knowledge no such studies exist for Bulgarian and Lithuanian. On the one hand, it is interesting to note that two Slavic languages are compared to a Baltic language (Lithuanian). On the other hand, the three languages are marginally present in the EU because of the later ascension of the three countries to the EU. The paper shortly describes the first electronic Bulgarian - Polish - Lithuanian experimental corpus, currently under development only for research. They also focus their attention on the morphosyntactic annotation of the parallel trilingual corpus according to the Corpus Encoding Standard: they present a review of the Part-of-Speech (POS) classification of the participle in the three languages - Bulgarian, Polish, and Lithuanian in comparison to another POS, the adjective. They briefly discuss tagsets for corpus annotation from the point of view of possible unification in the future with some examples.