"Kateřina mě učí tři sta nových slov."

Translation: Kateřina is teaching me three hundred new words.

[a-m-j]

300 should be accepted, rather than 3 hundred

[VladaFu]

3 hundred is what Duolingo's system does. When they see "three", they automatically accept 3. Too bad this does not apply to "hundred". There is nothing we can do except manually accepting the numbers everywhere. So it is Duolingo's decision and you have to complain in the main troubleshooting forums.

I just recommend to write words everywhere.

[Lars1o2]

Agreed. It still isn't accepted.