The RimWorld on Steam is a story generator. The only thing which is exactly the same for all playthroughs is the beginning. You're taking control over three castaways. You and your team crashlanded on an unknown planet, far away at the edge of the universe. They have to build the colony. While the story itself is told by three narrators - Cassandra Classic, Phoebe Chillax, and Randy Random - each has different traits in the context of the storytelling. Each event is completely random. Thus, each playthrough is going to be different, throwing you against new challenges.
Every thunderstorm, pirate raid, and traveling salesman is a card dealt into your story by the AI Storyteller. As it was mentioned before, each narrator has a different style. Cassandra, as her name suggests, provides you with the classic type of the challenge. She starts slowly but as you progress, the game is getting harder. Phoebe is the perfect choice for new players. Her story is usually relatively easy and the events work in your favor. Last but not least is Randy. He is going to take you on a rollercoaster where nothing has any sense. Do you know that RimWorld is using a complicated model of AI which analyzes your situation and creates events which are going to make an interesting story? The game is going to react to your actions.
Yes, it is true that you start only with three people. However, each one of them is going to have different traits. There are cooks, soldiers, criminals, aristocrats, accountants etc. Their previous occupation is quite important. For example, aristocrat is able to haggle with traders. On the other hand, he is not going to work on farm. There are also character traits. For example, a nervous person has higher efficiency during work. However, they are more susceptible to mental breakdowns. You have to learn how to utilize these elements.
Your survivors are not explorers. They are not Ellen Ripley. You are dealing with unfortunate survivors from a passenger liner destroyed in orbit. Thus, the situation in which they found themselves is really hard. You have to focus on the morale, needs, health or even relations with other colonists. The simple flu might kill half your colonists if you are not careful enough. Especially during a cold winter.