From Left to Right:
Mabel, Chara, Frisk.

From Left to Right:
Dipper, Dib, Asriel.
A fun fact about this piece: This is actually a sketch of three characters from a role-play website I'm on. I'd been aiming for three characters that knew each other but I couldn't think of another male child/ young teen who knew Dipper and Dib (Asriel hasn't met either of them.) until just recently. So I erased Asriel and drew someone else in his place:

Gideon.
And I have to admit, my drawing style doesn't do him much justice.

This is a picture of all the characters Undertale on the site.
From left to right: Underfell Flowey, Toriel, Chara, Asriel, Frisk, Sans, and Flowey.

And finally, a couple sketches made as a joke. Chara has three brothers to deal with xD Two flowers and the goat child himself.