dreihundert
three hundred
The cardinal number 300, consisting of three times one hundred.
Das Buch hat genau dreihundert Seiten.
The book has exactly three hundred pages.

three hundred
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three hundred
The cardinal number 300, consisting of three times one hundred.
Das Buch hat genau dreihundert Seiten.
The book has exactly three hundred pages.

German hundreds are built logically: you combine the basic number (here: 'drei' (three)) with the word 'hundert' (hundred). Both parts merge into a single word.
Der Saal bietet Platz für dreihundert Personen.
The hall has room for three hundred people.
Cardinal numbers under one million are always written as one word in German. So it is 'dreihundert' (three hundred), not 'drei hundert'. This also applies to more complex numbers like 'dreihundertfünf' (305).
Die Miete beträgt dreihundert Euro pro Monat.
The rent is three hundred euros per month.
When 'dreihundert' (three hundred) comes before a noun, it is not declined. The numeral keeps its form in all cases (nominative, accusative, dative, genitive).
Ich habe mit dreihundert Gästen gerechnet.
I planned for three hundred guests.
thirty
'dreißig' and 'dreihundert' are both numbers, but they are very different in size. 'dreißig' means 30, while 'dreihundert' means 300. Learners may confuse them because both begin with 'drei'. The second part is important: '-ßig' refers here to tens, while 'hundert' refers to hundreds. So a small listening or writing mistake can make a number ten times larger.

Das Fahrrad kostet dreihundert Euro.
The bicycle costs three hundred euros.
Für den Kurs haben sich dreißig Personen angemeldet.
Thirty people have registered for the course.