Why i get to fight a trio when im a solo player?

Because Riot does not care about Solo players.

The down vote are likely due to a large number of boards folks sick of Threads about DQ. As for why you get placed vs qued folks vs non qued folks. Trio is the new duo for when people want to run bot lane, have to take 3 to ensure you get adc support, Your team likely had a duo or trio on it as well due to how it sets teams up. Personally I dont really care all to much about playing vs or with groups, Keep teammates alive if i can and make enemy team dead when ever possible remains my goal.