There are three words for 'it' in German - er/sie/es. Which one should I use?

That's right, German has three words for English 'it'! But don't worry, there's a rule which lets you pick the right one! German nouns have grammatical gender and can be masculine (der), feminine (die), or neuter (das). You use 'er' to refer to masculine nouns, 'sie' for feminine nouns', and 'es' for neuter nouns:
der Hund is groß -> er is großdie Blume ist schön -> sie ist schöndas Haus ist riesig -> es ist riesig