The times that split the three groups in the booking page cannot be adjusted. Bookeo uses an algorithm - looking at the opening hours of your business - to best split the times in multiple columns.
However, you can hide the headings Morning, Lunchtime and Afternoon in your booking page:
1. Go to Settings>Colors and styles
2. In the Custom CSS section, paste
.cb_bhf_periodheading { display: none }
3. Save