Is it possible to have an ending on the 3rd beat of a measure?
Welcome to the forum.
If this question means, can you have a chord play on the third beat of a measure and hold that chord, Yes.
For example, let's say you want a Cm7 chord on beats one and two, then a Fmaj7 chord on beat three that is to hold. You would put Cm7,FMaj7... in the cell. Then be sure to leave several empty bars after in your song, for the chord to decay.
Or, if you wanted the FMaj to be a quick hit (what BIAB calls a Shot), then use two periods instead of three. One period means don't play, two is a shot, and three is a hold.
I hope I understood your question correctly. Does that help?