Summarize:

A box bay window has two side panels that meet at 90 degree corners with the back panel. Stretch your tape measure across the back window panel from corner to corner at the panel's top, middle, and bottom, making sure to keep the tape level. Record the smallest measurement you get and subtract ⅜ of an inch (1 cm) for some extra “wiggle room.” “Face-fix” simply means that the blind brackets will be attached on the front face of the window frame, and will normally conceal both the frame and the glass. As with the back panel, take 3 horizontal measurements for each of the side window panels. Use the smallest measurement for each panel as your guide. Then, subtract ⅜ of an inch (1 cm) for your “wiggle room.” Before you buy or order blinds, contact the manufacturer or refer to the packaging to get the precise dimensions of the brackets. You need to know the depth in particular in order to get the proper width measurements for all of your blinds. For example, your brackets might be 2.5 inches (about 6.5 cm) deep. Your back blind can span from corner to corner (call this Option A), in which case you need to subtract the bracket depth (e.g., 2.5 inches) from both of the side panel measurements. Or, the side blinds can span from corner to frame edge (Option B), in which case you need to subtract the bracket depth twice (e.g., 5 inches) from the back panel measurement.  When pulled down, either the side blinds will slightly overlap the back blind at the corners (Option A), or the back blind will overlap the side blinds a bit (Option B). To avoid any overlap, you can subtract 5 inches (for example) from the back panel and 2.5 inches from each of the side panels. But your measurements and installation will need to be extra precise to limit gaps in blind coverage.
Measure the width of the back window from corner to corner. Measure both side panels from their corners to the edges of their frames. Confirm the depth of the roller blind brackets you'll be using. Choose which measurement(s) you want to subtract your bracket depth from.