Get a third chibi once you have 10 spare star emblems, you buy 100 medals and then you can scout whatever girl you like (and has chibi) there.
You level them up and dress them for events, you need to do that for every event separately.
To dress them for an event you go to puchi organisation (ぷち編成) - the 3 smiling dolls, then you click Event (イベント), fill all 3 slots on the bottom and then under the recommended costume menu (お勧めぷち衣装設定) click the all at once (一括) button and it will equip them with the best clothes you have, you should see the final number and the modifier above, should be at least few thousand for you

My Live unit does about 300k (all 20 members) and as you can see with 3 fairly average equipped chibis (still not max level) I get around 80k bonus so its definitely worth it.
You also want to spend technic points on stat boosts, that's under the star icon - also in there you unlock special episodes for that character, extra comments and gestures