Translation:Kateřina is teaching me three hundred new words.
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.
Perhaps because she's teaching you the words right now, not repeatedly. Not sure though, it's always hard to decide whether the translation using present simple should be accepted, since there's only one present tense in Czech.