"Blinds" is a catch all term on this site. Do you want wove wood blinds, cellular shades, roller blinds, roman shades?

You need to decide if you want one wide or three different window coverings for this opening. Measure the entire width and height to be covered. If you want three coverings measure each of the side windows and also the middle one.

Next I want to tell you to take the measurements to a reliable store where allowances will be made to ensure the blinds will not touch each other when opening or closing. I'm not sure how that is accomplished now with internet sales. Because you want to hang the coverings outside mount you will have a margin to 'fudge'. There used to be hardware to replace the long interfering cranks you have. Use a metal measuring tape. Wood yardsticks are not always accurate. *~)