Is solo play meant to use just 3 heroes?
You are supposed to play as a single player, i.e. with 3 Heroes one your side and drawing a single card from the encounter deck each turn.

The pre-constructed decks in the game are unfortunately not the best and at the same time, the game is quite hard (even the beginner scenarios). Both combined can lead to your frustrating experience.

Here is a link to a guide for solo players who are new to the game: 
https://boardgamegeek.com/blogpost/16397/guide-new-lotrlcg-solo-players 
This should help you in getting started and get over the steep learning curve, which this game definitely has.