Hahahaha, I wish I was paid by WB.
In all seriousness, when a character is a draw, she's a draw. Margot Robbie's Harley Quinn wasn't only the breakout character of Suicide Squad, she was the breakout character in pop culture in 2016, and it doesn't seem as if she's slowing down. She's DC's Deadpool. The only problem I have is that DC/WB didn't strike while the metal was hotter. If they're exploring a trilogy around the character, she'd only be the second woman at DC to get one (Wonder Woman being the first).
I think it shows confidence in what they have with the BOP script, and it's hella cheaper to pump out three street level films and turn a profit than a huge tentpole. Walter Hamada is not one to waste money, and he already has a $1B franchise on his watch. As soon as Wonder Woman 1984 passes $200M at the box office, that will be another $1B franchise he's overseeing. Also, I think Margot Robbie has a keen mind for producing these films.