You can skip S3. Watch S4.
It's kind of true. As much as I love Kara and Mon-El, he wasn't written so well. I love him as a character, but not really their relationship and progression on the show. They are cute as a couple in real life though! Kara needs to focus on being Supergirl. Love shouldn't touch our superheroes. lol They don't know how to handle them. Like Clark and Lana in SV.
She has matured already. In S4.