paragraph question

I currently have the three sentences below in one of my chapters. Joseph is the POV character. Alexander is his father.

Is it better to write it as one paragraph?

Joseph walked around the back of his mother’s desk and stood behind her. Alexander walked over and placed himself next to her, both shielding their son. He looked over their shoulders, ready for… what?

Or three?

Joseph walked around the back of his mother’s desk and stood behind her.
Alexander walked over and placed himself next to her, both shielding their son.
He looked over their shoulders, ready for… what?

I lean toward the former. Granted, Alexander is the subject in the middle sentence, but all three sentences are very closely related (they're preparing for intruders who are trying to break through the study's doors).

Thanks
Dirk