In that image on the bottom, Streetlights is the long snakey building at the top. The new parking garage is shown as the yellow structure.
The new parking garage will occupy the location of the parking lot on the right side, south of the Hyatt.
Streetlights at Barton Springs will occupy the site of the parking lot on the left side, south of that small white building.
The Broadstone on Riverside will face Riverside and South First Street at the site of Runtex (red building).
So that still leaves open the possibility for three more buildings on the edges of that super block. One to the west of the Hyatt in that large parking lot, one southeast of the Hyatt and east of the new parking garage. The third tower would be south of the new parking garage facing Barton Springs Road.
I don't think those three towers are talking about any specific proposals, just more of a master plan of what can be built there.
That parking garage would have a heck of a view with all those buildings surrounding it.