The big announcement for Legends 3 so far is the introduction of a new character named Bullet (baretto). He seemingly will be a playable character, and exhibited the abilities to move quickly and run up walls. Other new characters and mecha were briefly shown as well. More footage is shown of Bullet performing a mission where he rids a town of Reaverbots. Aero is his Spotter. Bullet has a rushing kick attack, as well as seemingly a mid-air drop attack. In an additional mission, Bullet rides on an air bike which can attack. The environments truly are enormous. Additional brief footage was shown of Bullet fighting the winning Bonne boss mecha. It is uncertain for now if certain missions are divided between MegaMan and Bullet, or if you can choose who you want to use for each mission. All footage showed Bullet as the player character.
Kitabayashi also announced "Rockman DASH 3: The Prologue Game" for download on 3DS, for 200 yen. It will be available for download next month (May).
Unfortunately it appears my initial recording of the stream was corrupted, though I am taking from it what I can. For now, here are some images of Bullet. You may also find a few screens here.
Update: Here is a recording of the whole show. You can get to the DASH 3 footage at the two hour mark, which greatly expands on what was shown in today's prototype announcement. You can also check after the break for YouTube videos of the direct gameplay footage. Thanks MidniteW!
Thanks to Wizaman!